Description:
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From time to time, we see that users cannot confirm their subscription (or unsubscription). The first mail to request works fine, but when the +confsub part comes, it fails - users report they just get "Posting not allowed" instead of a confirmation, and in fact, the confirmation never happens.

For one user we could now nail the problem down to Evolution being used as mail client. Same server, same sender, but Thunderbird, and +confsub worked like a charm.

I lack further details, like Evolution version, but wanted to share in case someone has cycles free to investigate.