Getting SMTP Error 500
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
Reference Link : http://www.techbuddiesonline.com/pages/ms_outlook_exp.aspx