How to setup a Thunderbird IMAP account when user domain differs from mail server domain

In an attempt to setup an IMAP account in Thunderbird for my http://scalingcurve.at email address I eventually came to the conclusion that it is not possible.

What is the situation?

• My email address is robert.moestl@scalingcurve.at
• My email is hosted by Gandi, therefore the IMAP server is mail.gandi.net

What is the problem?

When I create a new email account in Thunderbird, I enter my full email address and the password. Then the setup dialog figures out the mail servers for IMAP and SMTP automatically. According to Gandi's mail client setup documentation all settings suggested by Thunderbird with respect to authentication etc. are correct. When hitting Done in the setup dialog, however, Thunderbird is "Checking the password..." and comes back with "Configuration could not be verified - is the username or password wrong?". After entering username and password fifty times I'm sure they're not. Somehow I manage to close the setup dialog and the account is created. I hit Get new messages for the newly created account. After a few seconds I'm requested to enter my password. There you can see it: if your enmail domain differs from the domain of the IMAP server, Thunderbird attaches the IMAP server name to your email address (which shall be the username). In my case the username ends up being robert.moestl@scalingcurve.at@mail.gandi.net.

So Thunderbird is simply providing the wrong username and I've not yet found a way to correct this behavior. I only found a Thunderbird bug report that resembles my situation.

Asking for help

If there's anyone who has resolved this issue, I'd be happy to hear from you.