Error: 521 The EMail is Blacklisted. - Some email servers verify the smtp server sending the email against a list of servers known to send unsolicited email (spam).
Error: 522 User over hourly mail quota - This may show up if the recipient has a quota on how many emails per hour can be received.
Error: 550 Invalid Domain. As per Microsoft: "This problem may be caused by a Domain Name System (DNS) record that points to the wrong mail host for a certain domain.". See the link below for more details. The link also resolves the "Relaying is prohibited" error message.
Error: 553 No such user here. - This message is returned when there is no recipient for the email address specified. It could be a spelling mistake or the account has been deleted.
Error: 553 sorry, that domain isn't in my list of allowed rcpthosts - May indicate that the IP address used on the Internet by the Exchange server is blacklisted (as potential source of unsolicited emails).
Error: 554 Quota violation - This may show up if the recipient has a quota on his/her mailbox and there is no more space to receive additional emails.
Error: 551 not our customer
Error: 553 malformed address
Error: 553 Invalid recipient address.
From a newsgroup post: "The errors reported are not errors of the Exchange server but they are being generated by the OTHER server. Exchange is merely telling you what error it got when it tried to send an email to that particular address."