Zendesk: SMTP user enumeration via mail.zendesk.com

Several methods exist that can be used to ██████████ SMTP to enumerate valid usernames and addresses; namely VRFY, EXPN, and RCPT TO. mail.zendesk.com does not reply to EXPN or RCPT TO so we will concentrate on VRFY in this report. The VRFY command will request that the receiving SMTP server veri...