You can in fact override the default sending address displayed. I use my default domain
user@default.com, collect their email via pop3 and then they use the zimbra account.
You'll kick yourself. Yes, fill in the reply too and tick it but it still doesn't override default, however, if you type the users email overtop of the from field
user@default.com can be replaced with
user@theirdomain.com.
Save and go.