Here is a list of possible "Reply codes" grouped by command

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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 501 Syntax error in parameters or arguments
• 503 Bad sequence of commands
• 421 <domain> Service not available, closing transmission channel

received data

• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 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 unrecognized
• 421 <domain> Service not available, closing transmission channel

QUIT

• 221 <domain> Service closing transmission channel
• 500 Syntax error, command unrecognized

TURN

• 250 Requested mail action okay, completed
• 502 Command not implemented
• 500 Syntax error, command unrecognized
• 503 Bad sequence of commands