SMTP Error 500 : The last command sent by your server
was not recognized as a valid SMTP or ESMTP command,
or is not formatted in the way the server expected. This
includes situations where the command is too long.
Note that commands that are recognized, but not
implemented, are handled by different status messages
(see 502 and 504).
Note : A “500 unrecognized command” server response
is often a case of antivirus software and/or firewall
interfering with incoming and/or outgoing SMTP
communications. Read your antivirus / firewall software
documentation thoroughly to solve the problem.
Examples of SMTP Error 500 error messages : “SMTP
Permanent Error: 500 Access Denied By Port Access” or
“SMTP Error 500 Line too long”.