Ability to filter messages into folders based on sender, keywords, etc. Able to comfortably handle thousands of messages. Over the years you'll accumulate tens of thousands of messages that may have significant archival value. If they're stuck in some obscure file format, they'll eventually become unuseable. Lots of shortcuts that you'll take advantage of with daily use.
Simplicity permits reliability.
Email software must be fast to use. Email Services and Addresses Changing your email address is a royal pain. Try to get an address that will last. Alumni organizations almost always offer email addresses to alumni, these will last as long as you pay up each year. You don't need to use the email address that your Internet Service Provider gives you.