SMTP failures on shared IP

[jbarney]

Did a CentOS 4.6 update this morning and am having major SMTP issues. Any suggestions would be extremely appreciated as I've had about 100 phone calls so far today.

By name works on a dedicated IP:

[jason@server ~]$ telnet mail.server.com 25

Trying X.X.X.X...

Connected to mail.server.com (X.X.X.X).

Escape character is '^]'.

220 server.com ESMTP

By dedicated IP works:

[jason@server ~]$ telnet X.X.X.X 25

Trying X.X.X.X...

Connected to server.com (X.X.X.X).

Escape character is '^]'.

220 firstscribe.com ESMTP


Shared IP or domain name on a shared IP doesn’t work:

[jason@server ~]$ telnet X.X.X.X 25

Trying X.X.X.X...

Connected to shared.com (X.X.X.X).

Escape character is '^]'.

 

[ziba@]

Hi,

Try waiting for some time after telneting to port 25 on shared ip address. SMTP welcome message should be displayed.

If it does not appear try checking routing preferences and make sure requests to these IPs are routed properly.

Such situations usually occur when there are too many awaiting connections on port 25.
You can check connections on this port and statuses of these connections with the following command:
# fuser 25/tcp
# netstat -anp | grep :25

If you are using DNSRBL checks for your mail server make sure that name servers defined in /etc/resolv.conf do respond and do not cause significant delays when processing dns queries. If any of them is too slow or does not respond try commenting it. It should speed up process of DNSRBL checks and thus decrease time in which smtp requests on port 25 are processed.