From Computer Tyme Support Wiki

Many of you are wondering "What did my server get blacklisted?" Unfortunately we live in a world on the internet that is both free and lawless and most email is spam. In the process of sorting it out innocent people are often victimized by criminals who hijack your servers. Sometimes in our efforts to fight spam we make a mistake and wrongly list you.

Nonetheless, here are some tips to distinguish you from spammers or to help you solve problems when you've been hacked. If you follow these practices you are less likely to have your email blocked.

If you got blacklisted then either you were sending spam or we made a mistake and thought you were sending spam. You may be sending spam and not know it.

Passwords

One way servers get blacklisted is because an attacker has figured out a weak password and is using one of your accounts to send out spam. The best way to reduce the possibility of this happening is to requite strong passwords. If you have setting to configure passwords you should require something more than just lower vase letters. Use MiXeD CaSe, numb3rs, spaces, and punctuation characters!