Mail Transfer Agent (MTA)

An MTA (Mail Transfer Agent) is server software that sends, receives, and routes email messages over the internet using SMTP (Simple Mail Transfer Protocol). MTAs handle the behind-the-scenes exchange of email between servers, moving messages from the sender’s domain to the recipient’s. 

When you send an email, your MTA connects to the recipient’s MTA and attempts delivery. Along the way, the message may pass through multiple MTAs – each logging, queueing, or routing the email based on policies and server availability. 

In the email infrastructure stack: 

• MTAs transmit messages 
• MUAs (Mail User Agents) are client-side apps (e.g., Outlook, Apple Mail) 
• MDAs (Mail Delivery Agents) store messages in inboxes 

For senders, MTAs: 

• Enforce authentication protocols (SPF, DKIM, DMARC) 
• Handle retries and error responses (e.g., bounces) 
• Are evaluated by inbox providers based on their sending reputation 

Mailgun operates as an MTA, offering both API and SMTP interfaces for sending.