No such thing as the "best" or "most secure". Email security is, for the most part, based on the user's intelligence and common sense. The good one's all have spam settings and junk settings and some level of security, but in the end it's up to your anti virus app and the user knowing not to open attachments or click on links unless he is 100% certain he knows what they are. I've been using Outlook since it first came out and couldn't do without it. A good, free alternative is Mozilla's Thunderbird.
Edited by Allan, 21 January 2012 - 11:36 AM.