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We are deploying some test e-mail servers with the subdomain mail (i.e., Now, it seems that Thunderbird discovers that e-mail server when we are configuring e-mail.

How does Thunderbird accomplish this? I'm assuming it tries to append a list of subdomain with the right hand side of your e-mail address. Is this correct? If so, does anyone have any documentation of this and/or the list of subdomains it tries in order?

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start here: – Zypher Apr 4 '11 at 19:53
I'm betting mail.yourdomain.blah is also an MX record for your zone. – John Gardeniers Apr 4 '11 at 21:37
It is not actually. But I think Zypher's comment hit it on the nail. Can you post it as an answer so I could mark it appropriately please. – Belmin Fernandez Apr 4 '11 at 23:18
Andrew did the hard work for me - he deserves the checkmark :) – Zypher Apr 5 '11 at 3:28
up vote 2 down vote accepted

As Zypher commented, according to (emphasis mine):

Try to find the config at the Mozilla server (if the email provider does not provide the configuration)

  • Fetch , e.g. . (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:

  • imap.domain
  • pop.domain
  • pop3.domain
  • smtp.domain
  • mail.domain

While it was once common for imap, pop/pop3 and 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.

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Thanks Andrew! Appreciate it. – Belmin Fernandez Apr 5 '11 at 15:13

I believe it query's Mozilla's servers which contain a database of common email services and the related settings and server addresses that those services use.

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