MTA
Definition
Mail Transfer Agent. The server that relays mail server-to-server. Postfix, Exim, Sendmail and Microsoft Exchange are the common implementations. Speaks SMTP.
- Full name
- Mail Transfer Agent
- Function
- Relays mail server-to-server
- Protocol
- SMTP
- Common implementations
- Postfix, Exim, Sendmail, Microsoft Exchange
Common questions
Why does the MTA matter in email header forensics?+
Each MTA a message passes through adds a Received header recording its hostname, IP address, and timestamp, so tracing the chain of Received headers back through successive MTAs is how an examiner reconstructs a message's real transmission path and detects spoofed origin claims.
How does an MTA differ from an MUA in the email delivery chain?+
The MUA is the client application the sender composes and reads mail in, while the MTA is the server infrastructure that actually relays the message between mail servers using SMTP, operating invisibly to the end user.
Can Received headers added by an MTA be forged?+
The originating client-supplied header fields can be forged, but headers added by legitimate MTAs the message actually passed through are generally trustworthy since each server appends its own header based on its own connection log, which is why the deepest untampered Received headers in the chain carry the most evidentiary weight.
Related terms
- DKIM
- DomainKeys Identified Mail. A cryptographic signature placed in a DKIM-Signature header. The signing key sits on the sending server; the public key...
- DMARC
- Domain-based Message Authentication, Reporting and Conformance. A DNS policy that stacks on top of SPF and DKIM, telling receivers what to do...
- MUA
- Mail User Agent. The client the user sees. Outlook, Thunderbird, Apple Mail, Gmail web, Spark, mobile apps. Speaks SMTP outbound and IMAP...
- Received Header
- A header line prepended by each mail server that accepts a message in transit, recording the server's own identity, the IP or...
- SPF
- Sender Policy Framework. A DNS TXT record on the sender domain that lists the IP addresses authorised to send mail for that...