======== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.4
- Linux/BSD distribution name and version: Ubuntu 14.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx):Apache
- Manage mail accounts with iRedAdmin-Pro? Free
- Related log if you're reporting an issue:
====

I've just installed iRedmail 0.9.4. If I create an alias under the Roundcube web interface I can send from this alias without any problem. But I I create the same alias under Thunderbird and try to send using IMAP I get the error:

An error occurred while sending mail. The mail server responded: 
5.7.1 <xxxx@yyyy.net>: Sender address rejected: not owned by user bbbb@yyyy.net.
Please check the message recipient "zzzz@gmail.com" and try again.

I'm not a programmer but I can change a configuration file. Can someone enlighten me as to what I need to change in order to send email from an alias using Thunderbird?

Thanks!