“Then there is the % address operator: user %domainB@domainA is first sent to domainA, which expands the rightmost (in this case, the only) percent sign to an @ sign. The address is now user@domainB, and the mailer happily forwards your message to domainB, which delivers it to user. This type of address is sometimes referred to as “Ye Olde ARPAnet Kludge,” and its use is discouraged“