Since many ISPs are trying to protect their users from being spammed; they usually refused to accept emails that are relayed from an IP Address--by performing a reverse DNS look-up...

To ensure that SMTP mail server, requesting to send emails actually exists and if it does not--mails may still be accepted--but gets deleted--without notifying the sender or after the initial HELO handshaking command... terminate the receiving process
(See: How email works) and your mails becomes undeliverable in your mail server outbox.
See: Email Filtering

If you are Blacklisted by ordb.org, MAPS and several others, you can get out of their list after you have stopped being an open relay mail server, by going to their site submit your mail server for testing.

Their automated system will perform the test and remove your IP address from their database.

Notes:
It is not (your) the mail server that gets blacklisted, it is your email domain IP address or the Block of IP address address that is maintained by your ISPs or administrators.

How to check if your email domain?
Desktop PCs mail server or your ISPs mail server is in the blacklist or not. Go to anyone of the following sites, follow the site instructions and you will get an immediate report...

http://openrbl.org/
http://www.ordb.org/lookup/
http://www.spamcop.net/bl.shtml
http://www.spambag.org/query.html
http://www.dnsstuff.com/

You can monitor some of the most widely-used blacklist at Google yourself at...(copy link and paste into your browser address bar).
http://directory.google.com/Top/Computers/Internet/Abuse/Spam/Blacklists/

or at MAPS Realtime Blackhole list http://mail-abuse.org/rbl/