The main problem after data security in mail services is spam. It is impossible to avoid spam messages coming from unexpeted sources, even if you are very carefull in the internet and don't leave your email address on some suspicious websites. However, even if you use it only for registration on some highly reputable services, it can be stolen by data gathering robots and then used for spam. One of the ways to get rid of spam messages is using Spamfence.
This software spam filter detects spam, analyzing the source of e-mail, header and message body. It is a self-learning program, so it can react on user's actions and 'remember' the rules according to which emails can be recognized as spam. Surely, it is not 100% effective, but among free spam filters it is the best. It defines suspicious messages with very low percentage of false positive results.
The main discomfort in using Spamfence is a need to have two or more emails accounts. Messages from one default address go to Spamfence, where they are analyzed for being spam or not. After filtering non-spam emails are sent to the second account that user actually uses.
The program is flexible in settings, very good in detecting spam, works with major email clients and absolutely free of charge.