if mails to or from you dont make it through it might be becoz of antispam blacklist ordb.org they closed service in 2006 but still many mailservers querie there servers. returns nothing until lately. but now they have finaly changed it and now ordb.org will reply "spam domain" for all requests! this is made to encourage admins to finaly change their spam checking routines and not to query the ordb.org servers anymore. what it means: if you or your host runs a antispam tool that queries ordb.org ALL mails will be marked as spam. google for "relays.ordb.org returning positive for everything" to get more info
I had experienced it for a server yesterday, it was quite a nightmare, forgotten to remove the ordb entry in the RBL
i think there will still be many who not get into solving this problem as quick as that. will start complaining about lost mails and all that.
Definitely, ordb was one of the most popular RBL during its times, I am sure a lot of legacy mail servers even hard code it into their systems.