Undelivered Mail Returned to sender? PRT(pointer) and A (address) mismatch

I am a gardener not a tech person, so be gentle…

I have set up e-mail on web mail and on mac0s 10.5.

Incoming is fine. Outgoing to gmail and yahoo works. When I send mail (either web or off mac) to another isp, I get a bounced e-mail msg.

Dreamhost says it is the other provider’s settings.

The other provider says:

Thank you for contacting us.
Our mail servers bounce any messages with mismatched PTR (pointer) and A (address)
records. This is in general, a way to prevent spam.”

Is this something I did wrong in set up?

I have followed (multiple times) the wiki on e-mail set-up for mac, always with the same result. Web mail fails, too. I am using port 587 with password authorization ON.

Any help would be appreciated.

Thank You