Is the recipient a local user, or a remote user. If it’s a remote user, then the administrator at the other end is blocking the email for some reason.
if it is remote then mrdenny is correct, the end recipient system is blocking email from your domain for some reason.
just to ask, does your ISP for your internet connection that your email server is using allow you to have a static IP address that your email goes out through? if not then more than likely your IP address has been added to the SBL, (SpamHaus Block List). this is only because it is a DHCP address. If you do get your IP address from DHCP then you can go to their website, and go to the delisting procedure link to have your IP removed from the list.
if you have a static IP you can still go to the site to check to see if it is on the SBL list or any of the other two lists that SpamHaus uses.