| CONNECT |
| 220 | <domain> Service ready |
| 421 | <domain> Service not available, closing transmission channel |
| HELO |
| 250 | Requested mail action okay, completed |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 504 | Command parameter not implemented |
| 521 | <domain> does not accept mail [rfc1846] |
| 421 | <domain> Service not available, closing transmission channel |
| EHLO |
| 250 | Requested mail action okay, completed |
| 550 | Not implemented |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 504 | Command parameter not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| MAIL |
| 250 | Requested mail action okay, completed |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 421 | <domain> Service not available, closing transmission channel |
| RCPT |
| 250 | Requested mail action okay, completed |
| 251 | User not local; will forward to <forward-path> |
| 550 | Requested action not taken: mailbox unavailable |
| 551 | User not local; please try <forward-path> |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 553 | Requested action not taken: mailbox name not allowed |
| 450 | Requested mail action not taken: mailbox unavailable |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 503 | Bad sequence of commands |
| 521 | <domain> does not accept mail [rfc1846] |
| 421 | <domain> Service not available, closing transmission channel |
| DATA |
| 354 | Start mail input; end with <CRLF>.<CRLF> |
| 451 | Requested action aborted: local error in processing |
| 554 | Transaction failed |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 503 | Bad sequence of commands |
| 421 | <domain> Service not available, closing transmission channel |
| 250 | Requested mail action okay, completed |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 554 | Transaction failed |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| RSET |
| 200 | (nonstandard success response, see rfc876) |
| 250 | Requested mail action okay, completed |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 504 | Command parameter not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| SEND |
| 250 | Requested mail action okay, completed |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| SOML |
| 250 | Requested mail action okay, completed |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| SAML |
| 250 | Requested mail action okay, completed |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| VRFY |
| 250 | Requested mail action okay, completed |
| 251 | User not local; will forward to <forward-path> |
| 550 | Requested action not taken: mailbox unavailable |
| 551 | User not local; please try <forward-path> |
| 553 | Requested action not taken: mailbox name not allowed |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 504 | Command parameter not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| EXPN |
| 250 | Requested mail action okay, completed |
| 550 | Requested action not taken: mailbox unavailable |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 504 | Command parameter not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| HELP |
| 211 | System status, or system help reply |
| 214 | Help message |
| 500 | Syntax error, command unrecognised |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 504 | Command parameter not implemented |
| 421 | <domain> Service not available, closing transmission channel |
| NOOP |
| 200 | (nonstandard success response, see rfc876) |
| 250 | Requested mail action okay, completed |
| 500 | Syntax error, command unrecognised |
| 421 | <domain> Service not available, closing transmission channel |
| QUIT |
| 221 | <domain> Service closing transmission channel |
| 500 | Syntax error, command unrecognised |
| TURN |
| 250 | Requested mail action okay, completed |
| 502 | Command not implemented |
| 500 | Syntax error, command unrecognised |
| 503 | Bad sequence of commands |