Cox doesn't block port 25 and this error isn't caused by that anyway. I've got my outgoing mail server set to mail.silveriafamily.com and I have it polling first to authenticate.
The information I've found regarding this error is:
Relay Access Denied
Another domain error that can be due to either the sender's or recipient's domain is a relay access denied error:
554 firstname.lastname@example.org: Recipient Address rejected:
Relay access denied
Another reason for receiving this error is the possiblity that a domain has recently changed hosts, and while the change has taken place, the new domain record has not yet propagated fully, and your message reached the old hosting company which no longer accepts mail for the leaving domain. If you receive this error, try resending your message twenty-four hours later.
So my concern is that for some reason email I send out is being bounced because it's trying to go through XO Hosting's mail server even though I've updated all my settings to have it go through mail.silveriafamily.com. Is there any way I can check this? Could the DNS update not have effected the mail server?