*** Bug 759678 has been marked as a duplicate of this bug. ***

Thanks for a bug report. Even the folder can be saved in settings, it's not used fully in the code. There is missing:
a) a place to select the Templates folder for the account (in Defaults tab)
b) use this account template folder to populate the popu-menu and
   to compose with the message from that particular account
c) 'Save as Template' should choose the Templates folder based on the From

The a) and the c) from the previous comment were not that hard, but the b), the menu populate from various accounts, was a challenge. I tried to make the code smart and not rebuild the menu often - actually rebuild it as less times as possible, similarly as it was done before, only more generic, due to multiple mail accounts being involved.

The popup menu in Templates also changed. If there are more accounts with available templates, then the first level is the account name to which the template belongs, otherwise only the template messages, with eventual subfolders, are shown. This has also the effect that if there are none messages in the On This Computer/Templates and only one (remote) account is set with a templates folder, then the first level under the Templates popup menu are direct messages from the other account, without sublevel with account names. I'm not sure whether it makes sense what I wrote, but once you'll see it in action you may see what I mean.

Created commit 10b70ae in evo master (3.21.2+)