SMTP trouble shooting : error 500 5.5.1 "Command unrecognized"

I am able to set up connection with mail server .I get response from mail server while sending HELO message. But I get error 500 5.5.1 Command unrecognized""  when I send all other messages. I am sure it is not syntax error that causing the problem.

Solution :


Try appending your messages with carriage return and line feed.
Example,

 "HELO mailserver_name\r\n";
"MAIL FROM:\r\n";
"RCPT TO:\r\n" 


In SMTP, error messages starting with 500 is due to syntax errors in messages sent. Focus on the message you are sending while you are debugging.If that does not work, using telnet, try to send the same messages and make sure that mail server responds correctly. Refer RFC 821 for further reference.

REPLY CODES BY FUNCTION GROUPS

500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented

211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]

220 Service ready
221 Service closing transmission channel
421 Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]

250 Requested mail action okay, completed
251 User not local; will forward to
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
451 Requested action aborted: error in processing
551 User not local; please try
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
354 Start mail input; end with .
554 Transaction failed