Long: sasl-authzid
Help: Use this identity to act as during SASL PLAIN authentication
Added: 7.66.0
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Use this authorisation identity (authzid), during SASL PLAIN authentication,
in addition to the authentication identity (authcid) as specified by --user.

If the option isn't specified, the server will derive the authzid from the
authcid, but if specified, and depending on the server implementation, it may
be used to access another user's inbox, that the user has been granted access
to, or a shared mailbox for example.