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? Code: Jan 20 11:05:35 ns sm-mta: AUTH=server, relay=[192.168.2.151], authid=info-a, mech=LOGIN, bits=0 Jan 20 11:05:35 ns sm-mta: o0K25TgT001551: from=<email@example.com>, size=329, class=0, nrcpts=1, msgid=<4B5664F5.firstname.lastname@example.org>, proto=ESMTP, daemon=IPv4, relay=[192.168.2.151] Jan 20 11:05:36 ns sm-mta: o0K25TgT001551: to=<email@example.com>, delay=00:00:01, xdelay=00:00:01, mailer=esmtp, pri=30329, relay=gmail-smtp-in.l.google.com. [18.104.22.168], dsn=2.0.0, stat=Sent (OK 1263953131 2si23121446pwj.33) What kind of role does Thunderbird play here? MUA or MTA?