SMTP 500 Syntax Error CMD Unrecognized

I have an Exchange 2003 mail environment with a Watchguard Firebox X2500 firewall. I have a few companies that can not send email to anyone in my company. Then there are other companies that intermittently can send email. Their email gets returned, but if they try again in a couple of hours then it might go through. The NDR usually states that “There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.

” These emails are received by the sender. I have no problem ever sending email to these companies. Any help on resolving this issue would be greatly appreciated. I’m currently working with Watchguard Technical support but they are kind of slow and haven’t found a solution yet. Thanks in advance.