Gmail no longer supports using google login credentials (2fa or otherwise) for accessing their services from applications. Their preference is OAuth2, but for clients that don’t support that yet (as is the case with K-9) you need to enable “less secure apps” (which is mostly a meaningless designation) and then create and use app-specific passwords. See: https://forum.k9mail.app/t/issue-on-gmail-authenticate-plain/3722 for more details, or search for this topic - it has been discussed in many threads in this forum.
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