Mail not coming through from some domains

Mails sent from certain domains are not coming through to the server while mails from most of the domains seems to come through. Below are some of the errors I can see:

some@e-mailaddress.com R=lookuphost T=remote_smtp:
SMTP error from remote mailer after MAIL FROM:
SIZE=1591: host some.host.header [123.456.78.9]:
553 hostname.mydomain.com does not exist

wcwd@hostname.mydomain.com :
unknown local-part &wcwd& in domain &hostname.mydomain.com&

anyoe got an idea to what could be wrong?

EDIT:
Managed to fix it but 99% sure I found a bug in CPanel that I will submit as soon as I maanage to describe it in writing.