Thunderbird replacement?


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My Thunderbird 60.0 (32-bit) is becoming increasingly slow and erratic, probably due to my elderly Windows 7 PC with nearly a million files in.
I am in the process of replacing the old PC with a new Windows 10 64-bit one.
Thunderbird has a wonderful extension: 'Show InOut' where the Inbox and Outbox are combined and threaded, the (i)ncoming messages displayed in (i)talics.
Another feature I must have is HTML support so that I can send forms for clients to return filled in.
Maybe someone could suggest another client with a least these two featutes?