I haven’t heard of this before — thanks for letting me know.

I suspect the reason is that VerifyRecipients works by connecting to an SMTP server and issuing “RCPT TO” commands for the purpose of getting the server’s success/failure response, but it stops short of actually sending an email.  This must be a tactic used by spammers to determine good/bad email addresses — and I hadn’t thought of that.   I’m going to add a note to the reference documentation to warn programmers to avoid this method.  You can instead call mailman.GetBadEmailAddresses() to get the failed email addresses (if any) after SendEmail is called.  The mailman.AllOrNone property controls whether SendEmail will return a failed status if  some of the email addresses were indicated “bad” by the SMTP server.

Here is a link to a page with related information about validating email addresses:
http://www.chilkatsoft.com/p/p_235.asp