As Zypher commented, according to https://wiki.mozilla.org/Thunderbird:Autoconfiguration (emphasis mine):
Try to find the config at the Mozilla server (if the email provider does not provide the configuration)
- Fetch https://live.mozillamessaging.com/autoconfig/emailaddressdomain , e.g. https://live.mozillamessaging.com/autoconfig/example.com . (Email address of user is not passed, otherwise Mozilla would have a list of email addresses of all users.)
- That file contains the mail configuration. Content is described on ConfigFileFormat.
- This service will have the configuration for all the major ISPs and email providers, so there's a 90+% hit rate
- It will not work for corporate email accounts.
- If a provider disagrees with a setting there, it can override the configuration by simply providing the config server in step 2.
Noting what I've emphasised above (which will be the case for your servers, as they won't be registered with Mozilla):
If we couldn't find any configuration file, we try to guess the configuration using some heuristics. We try imap.domain, pop.domain, pop3.domain, smtp.domain and mail.domain, and for each, try the common 2-3 ports. We check whether SSL is available, which authentication algorithms are announces by the server in the CAPABILITIES etc..
So in order it will try:
While it was once common for
smtp services to run on separate hosts (or at least common practice to have separate DNS records), the current practice is to run everything on a single
mail host, and that's what Thunderbird is trying and succeeding on.