Syntax error, command unrecognized (This may include errors such as command line too long)
The receiving SMTP mail server was unable to correct process the command(s) received. Also note, commands that are processed but not used will be handled with different response numbers.
The unrecognized command response is typically in regards to antivirus or firewall software. To confirm if this is the case, temporarily disable each one, re-instating them if not the area of concern.
SMTP Permanent Error: 500 Access Denied By Port Access
SMTP Error 500 Line too long