# What function is between MUA and MTA?



## goldenfire (Jan 20, 2010)

I have two machines to do this experiment.
One is 192.168.2.151 in which a Thunderbird is installed.
Another is 192.168.2.139 on which there is a Sendmail running.
Just send out a e-mail from the Thunderbird whose SMTP server is set to 192.168.2.139.
Then I got the following log messages at Sendmail server.
My question is why the mail submission from Thunderbird to Sendmail is considered as RELAY?
I think RELAY should be the relation between MTAs.
Could someone help?


```
Jan 20 11:05:35 ns sm-mta[1551]: AUTH=server, relay=[192.168.2.151], authid=info-a, mech=LOGIN, bits=0
Jan 20 11:05:35 ns sm-mta[1551]: o0K25TgT001551: from=<info-a@good.com>, size=329, class=0, nrcpts=1, msgid=<4B5664F5.80308@good.com>, 
proto=ESMTP, daemon=IPv4, relay=[192.168.2.151]
Jan 20 11:05:36 ns sm-mta[1553]: o0K25TgT001551: to=<goldenfire@gmail.com>, delay=00:00:01, xdelay=00:00:01, mailer=esmtp, pri=30329, 
relay=gmail-smtp-in.l.google.com. [209.85.216.58], dsn=2.0.0, stat=Sent (OK 1263953131 2si23121446pwj.33)
```

What kind of role does Thunderbird play here?
MUA or MTA?


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## dennylin93 (Jan 20, 2010)

MTA stands for mail transfer agent, and MUA stands for mail user agent.

The MTA is responsible for the transfer of emails between mail servers, while MUAs usually just retrieve mail from the server via IMAP or POP3 for the user.

Although some MUAs can send emails, they actually just send the email to the MTA so that it can be relayed to other servers.

Thunderbird, Mutt, Outlook, and Eudora are MUAs, and Postfix, Sendmail, qmail, and Exim are MTAs.


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## goldenfire (Jan 20, 2010)

dennylin93 said:
			
		

> MTA stands for mail transfer agent, and MUA stands for mail user agent.
> 
> The MTA is responsible for the transfer of emails between mail servers, while MUAs usually just retrieve mail from the server via IMAP or POP3 for the user.
> 
> ...



Thank you very much. I understood your words.
But from SENDMAIL log file, there is no different between receiving a mail from another MTA and from Thunderbird.
Sendmail treats them both as "relay=[192.168.2.151]".

So I wonder at least when Thunderbird submit mail, from the view of sendmail, Thunderbird is also a MTA.


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## DutchDaemon (Jan 20, 2010)

Sendmail doesn't care about the *actor* (MUA, MTA, shell), it cares about the *action*. The *action* in this case is receiving email from another actor with the intent to send it on to yet another actor. This action is called '*relaying email*', and it is akin to 'routing'. That's all there is to it.


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