Question : stat=User unknown in /var/log/maillog

I have installed a phpbb forum and now some users complain that they don't receive a confirmation message after they register. The problem is that many receive it properly and the problem seems to not consistent at all! I checked my maillog and found things like:

/var/log/maillog.1:Nov 21 12:21:29 server sendmail[28855]: jALHLTNW028855: [email protected], ctladdr=apache (48/48), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=31029, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (jALHLTs3028856 Message accepted for delivery)
/var/log/maillog.1:Nov 21 12:21:31 server sendmail[28858]: jALHLTs3028856: to=, ctladdr=ain.ws> (48/48), delay=00:00:02, xdelay=00:00:02, mailer=esmtp, pri=121121, relay=mx1.hotmail.com. [64.4.50.99], dsn=5.1.1, stat=User unknown


Since I don't care about receiving mails, I think I don't need to care about mx records for my server, right?
What else do I need to check? Is it necessary I do  some configuration for sendmail to use my hostname instead of localhost? This is almost a default RHEL4 installation.

Answer : stat=User unknown in /var/log/maillog

"Since I don't care about receiving mails, I think I don't need to care about mx records for my server, right?"

Yes, and no.

Yes, since you don't care about receiving, an MX record is not technically necessary for the host (altho one remains necessary for your Domain).

No, many anti-SPAM solutions will check whether the sending server is listed in DNS as an MX for the sending Domain, and if not, reject the connection, or otherwise delay or refuse to deliver the E-Mail. This is because most SPAM operations use botted Windoze boxes, or are hit-n-run via an open relay, and by checking for an MX record for the sending host, a lot of illegitimate senders can be filtered.

Bottom line: If you send E-Mail from a host, register it in DNS as an MX. Either that, or have it relay thru a host that is registered in DNS as an MX for your Domain.