SMTP 550 when sending to some domains

[PainOfDeath]

This problem only started when we switched from iMail to Exchange 2003. There are a few domains that won't accept mail from ours the user gets the following message in an NDR:

You do not have permission to send to this recipient. For assistance, contact your system administrator.
<exchange.ourdomain.local #5.7.1 smtp;550 5.7.1 Unable to deliver to <user@domain.com>>

I have checked various spam databases and we are not listed, and I used dnsstuff.com to see if there were any problems but everything comes back clean. I know the problem is probably on our server but how can I track it down? I am pretty new to Exchange so any help would be greatly appreciated!

 

[Zelandakh]

Check your MX record matches the IP address you send email out from. Also check your SPF record. Ensure you have a reverse DNS entry for your email server too.

 

[PainOfDeath]

It turns out that the e-mail address being used in one case was wrong, so there was no problem internally.

As I said, MX records and DNS is fine. I looked into creating an SPF record but I use DynDNS and am not sure how to add that record. Can anyone tell me how?