I've Zimbra 6.0.7.
One user need al external pop account.
I've configured it on preferences and almost all work.
Problem is From field.
User account for external pop is user+domain.it
so in From I set "First Last name" and "email@example.com".
On drop-down menu there are only zimbra email so I add correct email by hand.
Sending messare from this account "From:" field appear as username (user+domain.it) and not according with "From" field specification.