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