I continue to question why email clients with calendar, contacts, and tasks functionality don't have local sync integrated with Android or iPhone. Having said that, outside those "basic" integrated functions, my top 3 requirements to switch from Outlook are:

- Full encryption integrated into the client for all data (e.g., I don't want Windows Search able to index the mail so someone has access when the client is closed/unencrypted; I also remember testing Thunderbird years ago and was able to go into the individual unencrypted .eml files [I think that's what they were] and read the messages without having Thunderbird opened)

- Full local sync with Android/iPhone (i.e., home WiFi, Bluetooth, or USB cable); it still amazes me that Thunderbird still doesn't have this built-in

- Xobni-like functionality (e.g., showing all emails and attachments to/from sender when clicking on an email, keyword searches); yes, I know the plugin is still available but it doesn't work properly with current Outlook versions