Message ID

A Message-ID is a unique identifier automatically added to every email by the sending server. It’s formatted like an email address and looks something like this: 

<KIYjfnefhuw6ehtwj4nfj+reuihun00Khis-HIAhdHFBb714@mail.emailclient> 

This ID is used to: 

• Uniquely identify each individual message (even across mass sends) 
• Track message status and engagement via ESP tools like Mailgun’s Events API 
• Help mail servers thread replies and detect duplicates 
• Debug delivery issues by correlating logs with bounce and delivery data 

Each recipient gets a message with the same Message-ID if it’s a broadcast-style send, but systems may add unique IDs per recipient if needed for tracking. 

With Mailgun, the Message-ID lets you trace all events related to a message – like delivery status, opens, bounces, and clicks – across your email infrastructure. 

Note: When sending an email with contacts in CC and BCC, mail recipient email and CC/BCC emails will share the same Message-ID. When sending a mass email, it is important that each recipient has its own Message-ID. If one API call contains multiple recipients, we’d recommend that you use recipient variables – that way you’ll have a unique Message-ID for each recipient.