Some relative of mine forgot the password to access their e-mail account, but they were still remembering the pattern they used to pick it up, and that pattern could be easily represented by a regular expression. They did not have a backup email address to be able to use the password-reset mechanism.

[-] Delivery Failed and MX records

I was getting messages like the following form the MTA of my ISP(Alice/Telecom Italia) when sending email to some addresses: