ERror sending mail to particular domain from a particular domain

[evilphantom]

I know I posted this in the previous thread but I realized it was a different topic so here it is again

Okay I thought I fixed everything But I seem to have missed something here.. I am using a virtual user table and For one of my domains I recieve this error when sending to a particular domain:
Domain of sender address user@mydomain.com does not exist 501 5.6.0 Data format error.

Any Ideas. I can send from the other domain but not this particular one...

 

[evilphantom]

Turns out my OLD isp [who is currently hosting mail for domainX.com] has not yet removed the DNS entries for the domains in question. Also they didn't add an mx record for domainB.com but did for the others for somereason. ALl is okay now that I found the problem which will be fixed momentarily.