If in your Sendmail log (which is usually in /var/log/maillog) you get error like this one, or similar :
….collect: premature EOM: Connection reset by [10.10.10.11]
…. SYSERR(root): collect: I/O error on connection from [10.10.10.11]…
know that you have a problem with MTU establishement (maximum transmission unit), which is done through ICMP protocol (internet control message protocol).
Causes for this error may be :
***Bad LAN card on host, cable problems, switch problems – generally speaking connectivity
Test by pinging host from server, by using different MTU values (max is 1500) :
# ping -s 1500 10.10.10.11
PING 10.33.3.102 (10.10.10.11) 1500(1528) bytes of data.
1508 bytes from 10.10.10.11: icmp_seq=0 ttl=127 time=1.29 ms
1508 bytes from 10.10.10.11: icmp_seq=1 ttl=127 time=1.61 ms
1508 bytes from 10.10.10.11: icmp_seq=2 ttl=127 time=1.87 ms
See if the ping time is too big in relation to the ping time to some other related host (same network…) for which there are no problems – if it is, it is a network problem (if all other hosts are working well).
***See if ICMP protocol can go through firewalls and/or network devices – if not, let it, and test again.
***If the Outlook message is forwarded to many times (it is forwarded to the problematical host, it tries to forward it again) – it needs to be formed as a new mail message.