CramMD5ClientAuthenticator is documented here:

https://twistedmatrix.com/documents/19.10.0/api/twisted.mail._cred.CramMD5ClientAuthenticator.html

except that should be a private API.

It's re-exported in the __all__ of twisted.mail.imap4 and twisted.mail.smtp, but doesn't show up in either of those places.

I'm not sure what the actual issue is here (it might be that this is kind of a nonsense thing to do?) but we should probably do something more coherent, or at least report an error.