I have a customer that has two email servers, one server is sendmail the other is Zimbra 7.02.
They are sent an email (from a scheduled job) to addresses on both servers that originate from "user@domainxx.corp".
On the sendmail server the email is rejected, on the Zimbra server an error message is logged saying
Aug 20 21:34 mail named[3487]: too many timeouts resolving 'haxxx.na.domainxxx.corp/A' : disabling EDNS
but the email is delivered to the recipients mail box.

I have thought of 5 solutions so far.
1 add the senders domain name to the local dns table.
2 modify the sendmail configuration so that is does not do an nslookup.
3 add an email address on the Zimbra server for the recipient on the sendmail server, and add the account to his mail client.
4 switch the sendmail server to Zimbra.
5 get the sender to change the 'from' email address.

Your comments or suggestions would be appreciated.