Can't Receive emails

Hello all,
Errol here, I just set up a mail-cow server and installation went fine. My issue is I cannot receive email. I have checked DNS over and over. I double check all ports on server and still, I cannot receive emails.
The kick back I am getting is this:
_Your message wasn’t delivered. Despite repeated attempts to deliver your message, the recipient’s email system refused to accept a connection from your email system.
Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system is refusing connections from your email server. Give them the error details shown below. It’s likely that the recipient’s email admin is the only one who can fix this problem. _
Any help would be grateful…
Thank you, Errol…