To ease Thunderbird configuration for employees, a company may create http://autoconfig.emailaddressdomain/mail/config-v1.1.xml?emailaddress=user@emailaddressdomain. We have mailaddresses “username@domain” but the servers expect usernames “somenumber”, and according to the docs, the translation of those isn't implemented yet.

So we resort to PHP in /etc/apache2/sites-available/default:

<VirtualHost *:80>

  ServerName autoconfig.example.com
  ServerAdmin webmaster@localhost

<IfModule mod_php5.c>
AddType application/x-httpd-php .xml
</IfModule>


  DocumentRoot /srv/autoconfig/
  <Directory />
      Options FollowSymLinks
      AllowOverride None
</Directory>

  Alias "/mail/" "/srv/autoconfig/mail/"
  <Directory "/srv/autoconfig/mail/">
      Options Indexes MultiViews FollowSymLinks
      AllowOverride None
  </Directory>

</VirtualHost>
    

...and create config-v1.1.xml (not shown).

After this, if we fake the DNS through editing /etc/hosts, we only have to fill in name, mail address, and password when configuring Thunderbird. Now let's wait for the DNS to get updated...

Interesting further reading: on javascript autoconfiguration of Thunderbird.