I reluctantly switched to GMail about six months ago, after using many so-called “replacements for GMail” (the last of which was Fastmail). All of them were missing one or more features that I require of email:
Access to the same email on multiple machines (but, these can all be machines I control).
Access to important email on my phone (Android). Sophisticated access not important - just a high-tech pager.
Ability to organize messages by threads.
Ability to categorize messages by tags (folders are not sufficient).
Good search functionality.