In the comments on my Address Verification in Real Time Ken asks:
are there any legal issues in doing a large number of bulk SMTP transactions where you would connect issue say 20 RCPT TO commands to verify the user then send a RSET and QUIT. Immediatley connect again and do another 20 RCPT TO’s etc…
If you had say 250 thousand email addresses all pointing to the same MX record that would be you continually connecting 12500 times to cleanse your historical email list, then numbers would be small on forward flow as you would check them realtime as they came in.
Could you get into any serious trouble doing this?
I’m not sure there’s anything illegal about doing this. However, it is abusive and most mailservers will block IP addresses doing this.