Kostya,
I don't know, maybe this had been discussed long time ago, - before my time on this forum.
There is one feature that I am missing the most in Aquamail (compared to what's available in desktop- and shell-based e-mail clients): ability to change Fcc: for reach particular message from the default "Sent" folder.
In Thunderbird this is implemented via an extension "Copy Sent to Current":
https://www.ggbs.de/extensions/#cs2cIt allows you to choose the folder for each message individually (as well as to keep N most recent folders in the "short-list" of suggestions).
In Alpine, you can directly edit the "Fcc:" field in the headers of the message, and additionally, you can configure an entry in its internal address book to contain the default Fcc folder for each address (or by default it is saved to the folder with the same name as the address' alias).
E.g. the entry address saved as "kostya" will be saved to the folder "kostya", unless configured differently.
Because Aquamail relies on Android's Contacts app, saving of the default folders for each contact is not feasible here. But the ability to edit Fcc: field directly would be useful.
This allows to save the sent messages directly into specific folders.
I expect that users who use multiple folders would appreciate this feature.
Thanks in advance for considering this idea!