Como Zypher comentou, de acordo com link (grifo meu):
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.
Observando o que eu enfatizei acima (que será o caso de seus servidores, pois eles não serão registrados no 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..
Então, para tentar:
- imap.domain
- pop.domain
- pop3.domain
- smtp.domain
- mail.domain
Embora tenha sido comum os serviços imap , pop / pop3 e smtp serem executados em hosts separados (ou pelo menos a prática comum de ter registros DNS separados), a prática atual é executar tudo em um único host mail , e é isso que o Thunderbird está tentando e obtendo.