smtp: AUTH LOGIN should follow ietf-draft more closely

[wneessen]

Description

During the discussion in https://github.com/mjl-/mox/issues/223 it was discovered that we are not following the SMTP AUTH LOGIN draft closely, but rather expecting a "MAY" implementation detail for servers.

The draft states for clients:

The contents of both challenges SHOULD be ignored.

For servers it recommends:

There is at least one widely deployed client which requires that the challenge strings transmitted by the server be "Username:" and "Password:" respectively. For this reason, server implementations MAY send these challenge strings instead of those listed above.

We are following the server recommendation and expected "Username:", "username:" or "User name\x00" (the same for passwords).

To Reproduce

Try "SMTPAuthLogin" with a server that does not send one of the expected responses. The client will fail authenticate.

Expected behaviour

The client should follow the I-D and ignore the server responses (or at least not fail if a different response is sent)

Screenshots

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Attempted Fixes

No response

Additional context

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