This is the third Friday tip and maybe the best of them all J
To have a complete check on the customers external DNS when doing a mail project is maybe the best thing to do.
Very often I see that “everything” looks OK but when checking for PTR records they doesn’t exists.
This can be a problem, some receivers doesn’t accept the mail if the PTR records doesn’t match the mx record and the banner on the server.
With this link below you can search for the domain and check for blacklists, SMTP check and also the PTR check.