A few years ago I set up a Zimbra mail server behind Sophos UTM using Email Protection. Configuration for incoming mail went smoothly, but I ran into an issue with outgoing mail when using the UTM for relaying. It's been a few years so I'm a little foggy on the details, but the problem as I remember it was that the host name configured in Zimbra was mail.domain.com, which was the same name as the UTM SMTP relay. Because the two names were the same, when Zimbra sent mail to the relay it was rejected. The UTM and the mail server argued over having the same name. I resolved the issue by renaming the mail server to mail.domain.local so they didn't argue, and then set up a second domain on Zimbra for mail.domain.com. With this configuration everything works, but I've never been happy with it. I feel that there may be a more elegant way of doing things. Changing the SMTP hostname on the UTM doesn't seem like a good option since it's checked by receiving mail servers and should match RDNS and such. Perhaps I could leave the mail server name mail.domain.com and then modify the email header configuration in Zimbra to use a different name, but I've not looked into how that might be accomplished in Zimbra.
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