[Arnt]

You make it sound as if most mail servers log message contents. None of the servers I've used in the past 25 years did that (sendmail 5 on ultrix, later smail 3, then zmailer, then postfix).

The recipients' servers store the message with the password, of course, but they also store the other messages the same user has received from the same server, which in my case contain the same information as what could be accessed with the password. So the password offers very little additional value to an attacker, compared to just reading the mail.