We have remote clients that accessing their email via Thunderbird and IMAP.

They can receive emails no problems.

They can email other users on the same mail server with no problems.

They CANNOT however email users outside the server; and as you can appreciate this kinda renders the email server somewhat useless!

They are connecting to the SMTP server using authenticated details; yet that doesn't seem to make a difference.

What, as a Zimbra Administrator, do i need to do to allow my authenticated SMTP users to be able to send email while on the move?