I have 3 mail accounts in Thunderbird(T), had few problems in Windows 7 Pro. This happened when migrating to windows 10 Ultimate, since October 2015: Open T, loading down mail, program hangs, in short that is what I get of info, in error messages in T it is bad code or insufficient code; can be 3 problems, that can interact with each other. 1. bad code in Win10 or T. 2. Bad or harmful code in spam/mail. 3. Microsoft!!! (don't really like them)

I think you need debug info from mail clients to find problem. This problem is annoying, seem like problem is related to spam with bad code, but not sure. Spam changes from account all time, so first T hangs in downloading mail, may be when changing mail client in T downloading to 2. mail client or 3. It can be code in spam, not allowing delete or “being spam” before opened/read, when T is downloading from 2. or 3. it hangs because mail/spam then has to accept being handled as any mail. In fact I don't know – I just assume!