This is for Thunderbird.

Configuration server at ISP - or small company:

Given the email address "fred@example.com",
Thunderbird checks <https://autoconfig.example.com/mail/config-v1.1.xml?emailaddress=fred@example.com> (preferred) and
<https://example.com/.well-known/autoconfig/mail/config-v1.1.xml>
and the same URLs with http (see section SSL below).
The results are used in this order of preference.

If possible, please use <https://autoconfig.example.com/mail/config-v1.1.xml?emailaddress=fred@example.com>.