[Users] Feature: save sent mails in answered mails folder

[c.buhtz at posteo.jp]

In my working environment it would be very useful if mails wouldn't be
stored in the "sent" folder but in the same folder where the original
mail I reply to is stored.

But it would be important that this option would be related to a
specific folder. So I can turn it on for some but not all folders.

Some of my folders are representing person (e.g. "TheBoss"). And I view
the mails there threaded. So it would be great if my own replys would
appear in that threads.

Currently I have filters for that. But I have to create such filters
for each folder extra. A simple checkbox in the folder setting would be
great.

What do you think?