Various mail codes are returned by the SMTP Server which have different meanings. 

These codes and their meanings are defined below:

TPH1101 - Sender IP is on 

TPH1102 - Message is whitelisted

TPH1261 - Recipient is unknown

TPH1451 - Sender + IP has not been seen before and it is Greylisted

TPH1551 - The specified attachment is not allowed

TPH1552 - The specified mime type is not allowed

TPH1553 - Message is rejected to because of invalid or not allowed HELO used

TPH1554 - IP is listed in RBL

TPH1555 - Too many errors eg too many unknown recipients

TPH1556 - Message is considered to be spam

TPH1557 - Sender is not locally known

TPH1558 - Sender domain is not found

TPH1559 - 3rd party relaying

TPH1560 - Invalid characters found in the local_part i.e. first part of the email address

TPH1561 - Header Checks failed

TPH1562 - Message contained a virus

TPH1563 - Sender address is blocked by the recipient

TPH1564 - Sender host/IP address is blocked by the recipient