I'm not sure what you mean by a "custom" provider. Thunderbird can be used with accounts from any IMAP/SMTP e-mail service provider. Users are not restricted to Google and Microsoft.

not email provider, Oauth2 provider. For example if I secure my own email server with OAuth2.

I'm sorry that I don't know. Maybe someone else does. Have you set up your server? Can you do a test?

No, I can't do a test. The question is more directed if Thunderbird has the possibility to configure custom oauth providers. Every part of documentation I see is that Thunderbird just allows oauth2 on pre-known providers, namely Google and Microsoft, but there seems to be no visible way to edit oauth2 provider infos.

I just searched a bit deeper and found that there's already open issues about it, which answers my question

https://bugzilla.mozilla.org/show_bug.cgi?id=1809907 https://bugzilla.mozilla.org/show_bug.cgi?id=1866093

https://bugzilla.mozilla.org/show_bug.cgi?id=1809907 https://bugzilla.mozilla.org/show_bug.cgi?id=1866093

Someone is working on these. Unclear why the bug reports have not been updated.

Thunderbird Monthly Release 142 Recap https://blog.thunderbird.net/2025/08/thunderbird-monthly-release-142-recap/

Quotation from the above link:

Custom OAuth Support for Add-on developers Bug 1967370 ( https://bugzilla.mozilla.org/show_bug.cgi?id=1967370 ) New add-on API support for OAuth client registration now allows developers and organizations to add custom OAuth providers at runtime. Instead of requiring changes in Thunderbird’s core code, an add-on can handle the setup. Benefits:

• Supports custom OAuth providers through add-ons.
• Works with enterprise policies for organizational deployments.
• Simplifies integration with unique authentication systems.

Thunderbird Monthly Release 142 Recap https://blog.thunderbird.net/2025/08/thunderbird-monthly-release-142-recap/ Quotation from the above link:

Custom OAuth Support for Add-on developers Bug 1967370 ( https://bugzilla.mozilla.org/show_bug.cgi?id=1967370 ) New add-on API support for OAuth client registration now allows developers and organizations to add custom OAuth providers at runtime. Instead of requiring changes in Thunderbird’s core code, an add-on can handle the setup. Benefits:

• Supports custom OAuth providers through add-ons.
• Works with enterprise policies for organizational deployments.
• Simplifies integration with unique authentication systems.

do you know any add-ons that support this? I did not have luck yet..