Mailjet: Complete Technical Guide for Transactional Email in 2026

⚡TL;DR

• Mailjet (Sinch) processes 300+ billion emails/year with a modern v3.1 API limited to 50 messages per call.
• DKIM is mandatory: mailjet selector, 2048-bit keys by default since April 2024, 4096-bit available.
• SPF alignment fails by default (Return-Path on Mailjet domain): rely on DKIM alone for DMARC.
• Dedicated IP included from Premium 100k plan ($95/month) with mandatory 2-6 week warm-up.
• Free plan: 6,000 emails/month but limited to 200/day, ideal for dev/staging.

Introduction

Mailjet has established itself as a major player in transactional email in Europe, founded in 2010 in Nantes before being acquired by Sinch in December 2021 for $1.9 billion. Today, the platform processes more than 300 billion emails per year and serves over 100,000 paying customers in 150+ countries.

But behind the marketing interface and drag-and-drop editor lie critical technical subtleties: differences between API v3 and v3.1, DKIM configuration with key size choices, DMARC alignment failing by default for SPF, strict Free plan limits (only 200 emails/day), and dedicated IP constraints with mandatory warm-up.

This guide is intended for developers, DevOps, and system admins integrating Mailjet for transactional email. We detail the blocking points, critical configurations, and technical choices based on your stack and volumes.

Mailjet in the Sinch Ecosystem

History and Acquisitions

Mailjet was founded in 2010 in Nantes by Wilfried Durand and Julien Tartarin through eFounders (now Hexa). The acquisition timeline:

Date	Event
February 2011	Public launch
October 2019	Acquisition by Mailgun (Thoma Bravo)
December 2021	Acquisition by Sinch for $1.9B
2025	300+ billion emails/year

Positioning vs Mailgun

Within the Sinch ecosystem, the differentiation is clear:

Platform	Target	Strength
Sinch Mailjet	Marketers, SMBs	Intuitive interface, collaborative editor
Sinch Mailgun	Developers	API-first, 20+ APIs, advanced SDKs
Sinch Email on Acid	Email QA	Testing and cross-client preview

Key figure: during Black Friday 2025, Mailjet and Mailgun delivered 20.4 billion emails in November (+32% YoY).

REST API: v3 vs v3.1

Mailjet offers two versions of its Send API. The v3.1 is recommended for new projects.

Key Differences

Feature	v3	v3.1 (recommended)
Endpoint	POST /v3/send	POST /v3.1/send
Payload structure	Recipients array	Messages array mandatory
Property naming	Dashes (Html-part)	CamelCase (HTMLPart)
Messages per call	100	50
Variables	Vars	Variables
Templates	Mj-TemplateID	TemplateID
Response	Simple status	Detailed with MessageUUID, MessageHref

v3.1 Request Structure

{
  "Messages": [{
    "From": {"Email": "sender@captaindns.com", "Name": "Sender"},
    "To": [{"Email": "dest@captaindns.com", "Name": "Recipient"}],
    "Subject": "Order confirmation {{var:order_id}}",
    "HTMLPart": "<h1>Thank you {{var:firstname}}</h1>",
    "Variables": {"firstname": "John", "order_id": "12345"},
    "TemplateID": 12345,
    "TemplateLanguage": true
  }]
}

API Authentication

Authentication uses Basic Auth with:

• Username: API Key (public)
• Password: Secret Key (private)

curl -X POST https://api.mailjet.com/v3.1/send \
  -u "$API_KEY:$SECRET_KEY" \
  -H "Content-Type: application/json" \
  -d '{"Messages": [...]}'

Recommendation: rotate the Secret Key every 90 days.

Official SDKs

Language	Package	Installation
PHP	mailjet-apiv3-php	composer require mailjet/mailjet-apiv3-php
Node.js	node-mailjet	npm install node-mailjet
Python	mailjet-rest	pip install mailjet-rest
Go	mailjet-apiv3-go	go get github.com/mailjet/mailjet-apiv3-go/v4
Java	mailjet-client	Maven com.mailjet:mailjet-client:6.0.0
Ruby	mailjet-gem	gem install mailjet
C#/.NET	Mailjet.Api	NuGet Mailjet.Api

Important limitation: Mailjet does not support the send_at parameter for transactional emails. Scheduling is only available for marketing campaigns via /campaigndraft.

SMTP Relay

SMTP relay remains relevant for existing infrastructure and legacy CMS.

Configuration

SMTP Server: in-v3.mailjet.com
Ports: 25 (STARTTLS), 465 (SSL/TLS), 587 (recommended), 2525 (alternate)
Username: [API Key]
Password: [Secret Key]
Encryption: TLS supported on all ports

X-Mailjet-* Headers for Advanced Features

Header	Usage
X-MJ-CustomID	Custom identifier for tracking
X-MJ-EventPayload	Payload returned in webhooks
X-MJ-Campaign	Message grouping
X-MJ-TemplateID	Mailjet template ID
X-MJ-TemplateLanguage	Enable template engine (true)
X-MJ-Vars	JSON variables for personalization
X-MJ-TrackOpen / X-MJ-TrackClick	Tracking override (0/1)

SMTP vs API: When to Choose What?

Prefer SMTP if:

• Existing infrastructure (Postfix, Sendmail)
• CMS without SDK (legacy WordPress, Drupal)
• Systems without native HTTP capability

Prefer API if:

• New developments
• Need for immediate responses (MessageUUID)
• Advanced personalization (templates, variables)
• Fine-grained error handling

DKIM Configuration: The Critical Point

Why DKIM is Mandatory

Since the Gmail/Yahoo requirements of February 2024, DKIM authentication has been essential. Without DKIM configured, your emails risk going to spam or being rejected.

Configuration Process

1. Account Settings → Domains & Sender addresses → SPF/DKIM Authentication
2. Add the domain if not listed
3. Click Setup SPF/DKIM Authentication
4. Copy the DKIM values provided
5. Create the TXT record in your DNS
6. Click Refresh to verify

Mailjet DKIM Parameters

Parameter	Value
Selector	mailjet
DNS Hostname	mailjet._domainkey.captaindns.com
Type	TXT (not CNAME)

Available Key Sizes

Size	Status	Recommendation
1024 bits	Legacy	Maximum compatibility
2048 bits	Default since April 2024	Recommended
4096 bits	Available (new accounts)	Maximum security

Example DKIM record:

Type: TXT
Host: mailjet._domainkey
Value: k=rsa; p=MIGfMA0GCSqGSIb3DQEBA...[public key]...QIDAQAB
TTL: 300

Key rotation: available via Account → Domains → Regenerate Key. Requires DNS update after regeneration.

SPF, DKIM and DMARC Alignment

The SPF Alignment Problem

By default, Mailjet uses its own domain for the Return-Path (bnc3.mailjet.com). Consequence: SPF alignment systematically fails for DMARC.

How DMARC Works with Mailjet

Method	Authenticated Domain	From Domain	Alignment?	DMARC Result
SPF	mailjet.com (Return-Path)	captaindns.com	No	Does not contribute
DKIM	captaindns.com (d= signature)	captaindns.com	Yes	DMARC passes

Strategy: rely on DKIM alone for DMARC compliance. A single aligned mechanism is enough.

Enable SPF Alignment (optional, paid accounts)

If you want SPF alignment (double DMARC validation):

1. Create a CNAME: bnc3.captaindns.com CNAME bnc3.mailjet.com
2. Submit a support ticket to Mailjet with DNS screenshot and API key
3. Mailjet enables custom Return-Path (limited to one per API key)

Note: this configuration is reserved for paid accounts and requires support intervention.

Recommended DMARC Configuration

# Phase 1 - Monitoring
_dmarc TXT "v=DMARC1; p=none; rua=mailto:dmarc@captaindns.com"

# Phase 2 - Quarantine
_dmarc TXT "v=DMARC1; p=quarantine; pct=100; rua=mailto:dmarc@captaindns.com"

# Phase 3 - Reject
_dmarc TXT "v=DMARC1; p=reject; pct=100; rua=mailto:dmarc@captaindns.com"

Dedicated vs Shared IP

Thresholds for Dedicated IP

Criterion	Threshold
Absolute minimum	100,000 emails/month
ISP recommendation	150,000 emails/month + 5,000/day
Included free	Premium 100k+ and Custom plans

Warm-up Process

A new dedicated IP requires mandatory warm-up:

Parameter	Value
Type	Manual, guided by Mailjet deliverability team
Start	~300 emails day 1
Progression	+20% per day
Duration	2-6 weeks depending on volume and engagement
Contact	Support/sales mandatory (no self-service)

When to Stay on Shared IP?

Stay on shared IP if:

• Volume below 100,000 emails/month
• Irregular or sporadic sends
• Pure transactional at low volume
• No resources to manage warm-up

Switch to dedicated IP if:

• Regular volume above 150,000 emails/month
• Need to separate marketing and transactional traffic
• Whitelisting or compliance requirements
• Resources to monitor reputation

2025 Pricing

Available Plans

Plan	Price/month	Emails/month	Emails/day	Dedicated IP
Free	$0	6,000	200	No
Essential 15k	$17	15,000	Unlimited	No
Premium 15k	$27	15,000	Unlimited	No
Premium 100k	$95	100,000	Unlimited	Included
Premium 500k	$470	500,000	Unlimited	Included
Custom	Quote	Custom	Unlimited	Included

Annual discount: 10% on all plans.

Features by Plan

Feature	Free	Essential	Premium
APIs, SMTP, Webhooks	Yes	Yes	Yes
Mailjet logo	Present	Removed	Removed
Segmentation	No	Yes	Yes
Automation	No	No	Yes
A/B Testing	No	No	Yes (10 versions)
Multi-users	No	No	Yes
Dedicated IP	No	No	Yes (100k+)
SSO	No	No	Yes (100k+)

Additional Costs

• Overage: $2.24/1000 (low tiers), $1.04/1000 (high tiers)
• Email validations: 500/month (Essential), 2,000/month (Premium 100k+)
• Nonprofit discount: 20% on monthly plans
• No credit rollover: unused emails expire at end of cycle

Technical Limits

Rate Limits

Element	Limit
Test Mode accounts	10 emails/hour
Free plan	200 emails/day
SMS API	6 requests/second
Dedicated IP	~100,000 emails/hour
Messages v3 per call	100
Messages v3.1 per call	50

Throttling: HTTP code 429 Too Many Requests with error MJ-002: Exceeded API Rate Limit.

Platform Quotas

Element	Limit
Maximum email size	15 MB
Recipients per call (v3)	50
Message objects per payload (v3.1)	100
HTML size (before Gmail clipping)	102 KB
Image upload	2 MB (recommended less than 200 KB)
Contacts Free plan	1,000

Bounce Handling

Type	Mailjet Behavior
Hard bounce	Automatic block for 90 days
Soft bounce	Retry for 24h then marked soft-bounced
Spam complaint	Systematic block via FBL
Unsubscribe	Automatic removal from lists

Webhooks and Event API

Available Events

Event	Description
sent	Email accepted by destination SMTP server
delivered	Delivery confirmed
open	Open detected
click	Link click
bounce	Bounce (hard or soft)
blocked	Pre-blocked by Mailjet
spam	Marked as spam
unsub	Unsubscribe

Webhook Configuration

POST /v3/REST/eventcallbackurl
{
  "EventType": "open",
  "Url": "https://captaindns.com/webhook",
  "Version": 2
}

Retry logic: attempts every 30 seconds for 24h if non-200. URL suspended after repeated failures.

Security: HTTPS with Basic Auth recommended (https://user:pass@captaindns.com/webhook).

Template Language

Mailjet uses Jinja2/Twig-like syntax:

Variables

Mailjet offers four types of variables:

• API variable: var:name syntax for data passed via the API
• Default value: var:name:"default" syntax if the variable is absent
• Contact data: data:property syntax for persistent contact properties
• Predefined variable: mj:contact.email syntax for system data

Conditionals and Loops

Conditional:
{% if var:totalorders > "5" %}
VIP promo code: -10%
{% else %}
Welcome new customer!
{% endif %}

Loop:
{% for item in var:products %}
Product: item.name - item.price EUR
{% endfor %}

Compliance and Certifications

Certification	Status
ISO 27001	Yes (first pure ESP certified, 2017)
ISO 27701	Yes (privacy extension)
SOC 2 Type II	Yes (~400 controls)
PCI DSS	Yes
HIPAA	On request
GDPR	First company certified AFAQ GDPR (2018)

Data: exclusive storage in the EU (Google Cloud Platform).

Account security:

• Two-factor authentication (2FA) via TOTP apps (Google Authenticator, Authy)
• SSO and SAML on Premium 100k+ plans (Okta, Azure AD, Auth0)
• Login Challenge for unknown IPs (since March 2024)

Action Plan: Getting Started in 7 Steps

1. Create Account and Generate Credentials

• Create a Mailjet account (Free possible)
• Generate API Key + Secret Key in Account Settings → API Key Management
• Note both: API Key (public) and Secret Key (private, to protect)

2. Add and Validate Sender

• Create a sender in Account Settings → Senders & Domains
• Validate the address via the link received by email

3. Configure DKIM (mandatory)

• Go to Domains & Sender addresses → SPF/DKIM Authentication
• Copy the DKIM values generated by Mailjet
• Create the TXT record at your registrar:
  • Name: mailjet._domainkey
  • Value: the provided public key
• Wait for propagation (24–48h max)
• Click Refresh to verify

4. Configure SPF (optional)

If you want the SPF include:

v=spf1 include:spf.mailjet.com ~all

Note: the SPF include alone is not enough for DMARC alignment (see alignment section).

5. Choose Sending Method

Option A: REST API v3.1

• Implement the POST /v3.1/send endpoint
• Use SDKs if available in your stack
• Create templates in Templates if needed

Option B: SMTP relay

• Configure your app with:
  • Host: in-v3.mailjet.com
  • Port: 587 (STARTTLS)
  • User: API Key
  • Password: Secret Key

6. Configure Webhooks

• Account Settings → Event tracking (Webhooks)
• Create endpoints for: delivered, bounced, opened, clicked, spam
• Secure with HTTPS + Basic Auth

7. Test and Monitor

• Send a test email
• Check in Stats that sending is OK
• Check the received email headers (DKIM pass, correct From domain)
• Monitor the dashboard for metrics

FAQ

What is the difference between API v3 and v3.1?

The v3.1 uses CamelCase property names (HTMLPart) vs dashes (Html-part), limits messages to 50 per call (vs 100), and returns more detailed responses with MessageUUID and MessageHref. The v3.1 is recommended for new projects.

Why does SPF alignment fail with Mailjet?

Mailjet uses its own domain for the Return-Path (bnc3.mailjet.com). DMARC checks alignment between the domain authenticated by SPF and the From domain. Since these domains differ, SPF does not contribute to DMARC. Solution: rely on DKIM alone, which passes in alignment.

Do I need to add include:spf.mailjet.com in my SPF?

It's not mandatory and doesn't change DMARC alignment. The SPF include can be useful for basic SPF validation, but DKIM alone is sufficient to pass DMARC with Mailjet. SPF alignment is only possible with a custom Return-Path (paid accounts, support required).

Is the Free plan sufficient for a production app?

Yes, if your volume is 200 emails/day or less (note: the Free plan offers 6,000/month but is limited to 200/day). All transactional features (API, SMTP, webhooks) are available. Ideal for MVP, staging, or very low volume apps.

What DKIM key size should I choose?

2048 bits has been the default since April 2024 and suits the majority of cases. 4096 bits is available for new accounts if you want maximum security. 1024 bits is legacy and not recommended except for specific compatibility constraints.

When do I need a dedicated IP?

If you send more than 100,000 emails/month regularly, if you need to isolate your transactional traffic reputation, or if you have whitelisting requirements. Dedicated IP is included in Premium 100k+ plans and requires a 2-6 week warm-up.

How does dedicated IP warm-up work?

Warm-up is a manual process guided by the Mailjet team. It starts at about 300 emails/day and increases by 20% per day for 2 to 6 weeks. You must send daily to establish reputation. Support/sales contact is mandatory for activation.

Can I schedule transactional emails for a specific time?

No, Mailjet does not support the send_at parameter for transactional emails via the Send API. Scheduling is only available for marketing campaigns via the /campaigndraft endpoint. For transactional, you must handle scheduling on the application side.

Glossary

• SPF (Sender Policy Framework): Authentication protocol that lists servers authorized to send emails for a domain. SPF authenticates the Return-Path domain. With Mailjet, the include is spf.mailjet.com.

• DKIM (DomainKeys Identified Mail): Authentication protocol that cryptographically signs emails. Mailjet uses the mailjet selector and offers keys from 1024 to 4096 bits.

• DMARC (Domain-based Message Authentication, Reporting & Conformance): Protocol that verifies alignment between SPF/DKIM and the From header. With Mailjet, only DKIM passes in alignment by default.

• Return-Path (Envelope From): Technical address used to route the email and handle bounces. At Mailjet, it's bnc3.mailjet.com by default, which prevents SPF alignment.

• DMARC Alignment: Verification that the authenticated domain (SPF or DKIM) matches the From domain. Relaxed mode (default) accepts subdomains.

• API Key: Mailjet public identifier used as username for API and SMTP authentication.

• Secret Key: Mailjet private key used as password. Should be rotated every 90 days.

• Send API: Mailjet email sending endpoint. Version v3.1 recommended (POST /v3.1/send).

• Rate limit: Throughput limit. HTTP code 429 when exceeded. v3.1 message limited to 50 per call.

• Warm-up: Progressive ramp-up process for a dedicated IP. 2–6 weeks at Mailjet.

• Webhook: HTTP URL called by Mailjet on events (delivered, bounced, etc.). Retry for 24h if failure.

Official Sources

• Mailjet API Documentation
• Mailjet DKIM Configuration
• Mailjet 2025 Pricing
• Sinch Acquisition of Pathwire
• Gmail/Yahoo February 2024 Requirements