| 0 | UNDETERMINED - (ie. Recipient Reply) |
| 10 | HARD BOUNCE - (ie. User Unknown) |
| 20 | SOFT BOUNCE - General |
| 21 | SOFT BOUNCE - Dns Failure |
| 22 | SOFT BOUNCE - Mailbox Full |
| 23 | SOFT BOUNCE - Message Too Large |
| 30 | BOUNCE - NO EMAIL ADDRESS. VERY RARE! |
| 40 | GENERAL BOUNCE |
| 50 | MAIL BLOCK - General |
| 51 | MAIL BLOCK - Known Spammer |
| 52 | MAIL BLOCK - Spam Detected |
| 53 | MAIL BLOCK - Attachment Detected |
| 54 | MAIL BLOCK - Relay Denied |
| 60 | AUTO REPLY - (ie. Out Of Office) |
| 70 | TRANSIENT BOUNCE |
| 80 | SUBSCRIBE Request |
| 90 | UNSUBSCRIBE/REMOVE Request |
| 100 | CHALLENGE-RESPONSE |
