Question : Outgoing Emails to Certain Domains Rejected

For the past 2 months or so I've had a problem with some outgoing emails being rejected by the recipient domains stating " There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.  om #5.5.0 smtp;550-Your message was rejected by this user and was not delivered.>  These are people we've previously been able to email with no problems.  I'm sure this is a config somewhere in my end, either on the Exchange Server (Exchange 2003, single domain environment on Server 2003) or in the router (Sonicwall TZ 190 Wireless Enhanced) - maybe a NAT related issue?  I haven't changed anything on the Exchange server or in the router so I'm at a loss as to why this started happening.  Not really sure where to start troubleshooting so any expert advice would be appreciated.

Answer : Outgoing Emails to Certain Domains Rejected

we are concerned about the PTR records on external DNS not internal... check it this can be one of the reason