Admin console gui > COS > Preferences tab> Allow sending email from any address:

zmprov mc COSname zimbraAllowAnyFromAddress TRUE
Then in the accounts/persona you can type in the field:
Obviously not everyone wants to set AllowFromAny wide open- so vote for
Bug 12094 – Identities: Auto verify user settable from address >
Bug 29974 – persona/external account from field address verification
Admin console gui > Account > Preferences tab> Allow sending email only from these:

zmprov ma
user@domain.com +zimbraAllowFromAddress
other@external.com
Aliases (can be used for inbound delivery) are automatically included in the list of addresses a user can send as.
Then in accounts/persona settings you're restricted to actual, aliases, & specified:
