Welcome to the forums,
Visit your admin console gui > global or server sections > pop & mta tabs
What's the mail client program? ZCS version?
http://www.zimbra.com/forums/announc...r-profile.html
ie: You really need to list off what you have set currently for us to help you out further, plus I don't know if you're asking about incoming or outgoing...
-Anything you set in an individual server section will override the global defaults.
-Depending on how much you've changed you'll need to run a postfix reload, zmmtactl & zmmailboxdctl stop/start, or even a full zmcontrol stop/start afterwards to make sure things apply.