Sounds like a DNS issue. Be sure to read and understand this --> Split DNS
Originally Posted by thinhat
Before anyone can give you any help beyond a simple link to a tutorial / article, you are going to need to give more details than just a general description of the problem.
I've seen this all over the forums when members are trying to get more details in order to help so I'll copy/paste the same info here.
type the following commands and show us the output:
Exact product version:
Your short host name:
su - zimbra -c "zmcontrol -v"
Your long hostname:
DNS Info (use the hostname above in this command:
dig mail.mydomain.com any