0 = authentication failure
1 = SMTP relay denied
2 = SMTP relay allowed
3 = Unrecognised local recipient
4 = Recognised local recipient
5 = Message detected as spam
6 = Virus detected in message
7 = SMTP rule blocked the message
8 =
DNS Blacklist blocked the message
9 = SMTP syntax error (eg unrecognised command or RCPT TO command without a preceding MAIL FROM, etc)
10 = Message too big
11 = Message contained attachment blocked by the attachment filter
12 = Message contained a 'partial' attachment (partial attachments are considered dangerous and are extremely rarely used legitimately nowadays)
13 = The message was rejected due to an SPF error (see Services → SMTP → Spam Reduction → SPF section)