Extended system procedures for MAPI and SMTP use the following return codes.
| Return code | Meaning |
| 0 | Success. |
| 100 | Socket error. |
| 101 | Socket timeout. |
| 102 | Unable to resolve the SMTP server hostname. |
| 103 | Unable to connect to the SMTP server. |
| 104 | Server error; response not understood. For example, the message is poorly formatted, or the server is not SMTP. |
| 421 | <domain> service not available, closing transmission channel. |
| 450 | Requested mail action not taken: mailbox unavailable. |
| 451 | Requested action not taken: local error in processing. |
| 452 | Requested action not taken: insufficient system storage. |
| 500 | Syntax error, command unrecognized. (This may include errors such as a command that is too long). |
| 501 | Syntax error in parameters or arguments. |
| 502 | Command not implemented. |
| 503 | Bad sequence of commands. |
| 504 | Command parameter not implemented. |
| 550 | Requested action not taken: mailbox unavailable. For example, the mailbox is not found, there is no access, or no relay is allowed. |
| 551 | User not local; please try <forward-path> |
| 552 | Request mail action aborted: exceeded storage allocation. |
| 553 | Requested action not taken: mailbox name not allowed. For example, the mailbox syntax is incorrect. |
| 554 | Transaction failed. |
