Thanks Popowich, first I wanted to share this link with everyone, just in case you haven't ran into it. It makes viewing server logs very easy! Basically for those non-shell guys like me, you can use this plugin with your WHM via browser!
www.logview.org
My first question will always be what do the logs say?
If you check the logs on the server that isn't working are any problems being logged?
I sent another email but no errors came up on the logs - I am looking at the exim maillog - I look at other logs but found no errors
Do they show their mail server accepting the email reply from your server?
I am not sure where or how I could check for this? can you help?
Are you able to send out any email from that server?
Yes, I can send emails normally to everyone else it seems, except to them or my other client under a godaddy acct.
Can you send a new email to them through that server?
nope, not even a new email will go through
Are the From: and Reply-To: settings in their email set correctly?
not sure what you mean by this, I use my email all day long so I am guessing all these settings are correct.
Thank you