Don’t Blame Sendmail…
I’ve been having a hell of a time trying to figure out why I can receive some email and not others from my mail server. I noticed a week ago that emails were not getting through from certain domains and it was starting to drive me crazy not being able to figure it out. I’ve had a few problems with my server as of late, this being one of them. My company ended up moving offices on the 1st, so I initially wanted to blame my ISP (who wouldn’t?!?).
After spending quite a bit of time Googling and reading sendmail man pages I had finally learned enough to debug this. How many of you out there know that you can telnet to port 25 and send commands directly to the mail server? In the course of debugging I had figured out that I could receive connections on port 25 and the standard “handshaking” would happen (EHLO, etc…), but for some connections, it would get stuck on the DATA portion.
I finally ended up posting on comp.mail.sendmail. For some odd reason I needed top drop the MTU on my server from 1500 to 576 bytes.
To change the MTU, all you need to do is ifconfig eth0 mtu 576.
http://www.sendmail.org/faq/section3.html#3.10
http://www.sendmail.org/tips/pathmtu.html