Why not display a simple warning?

To register on various sites, we are usually asked to enter a mail address and password. And many users enter the same password that is used to log into the mail.

As a result, having a base of such users, you can freely look in the mailboxes of hapless users and send letters on their behalf (or, for example, write messages on other forums in other forums).

I suggest that all decent people take it as a rule on registration forms when entering a password (where they usually ask to enter a password twice) to display a warning like " Do not use the password from your mailbox! ".
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I think it will be fair in relation to the "teapots". After all, they write warnings like “the password must be at least 8 characters and contain numbers ...”.

PS It is clear that “experienced users” do not have this problem - either they use specially allocated mailboxes “that are not sorry”, or use different passwords, or periodically change the mail password (depending on the degree of paranoia).