Need to replace my thunderbird

Greetings.

I have been quite happily using thunderbird (well Postbox) to connect with my gmail, with addons for contacts & calendar.

Now, as you know thunderbird is being dropped like a hot potato by Mozilla, hence it will be soon older and decrepiter. Therefore I need to think about replacing it soon.

I'd like something (I could have a separate application for the calendar - I'm cool with that) that can replace all the functionality with thunderbird.

I don't regard Outlook or those "Put gmail in a cleverly disguised web browser" as a solution. Any ideas guys? There must be others in this situation?