That is the old C core, you should not use it anymore, current versions of UI’s won’t compile with it.
You can find the current core here: GitHub - deltachat/deltachat-core-rust: Delta Chat Rust Core library, used by Android/iOS/desktop apps, bindings and bots
The code area you are looking for is probably this: https://github.com/deltachat/deltachat-core-rust/blob/31e3169433eb9405b75ba4564517fd2690b76734/src/imap.rs#L1709 , but there could be additional code areas.
Why didn’t we add it already and are currently not very in favor of implementing custom folder names:
- Normal People don’t understand what these folders mean and complain after they changed settings: “I don’t receive messages anymore DC is broken” - turned out that user had accidentally enabled “watch only DeltaChat folder”
- Deltachat aims to have only settings where necessary, we try to find good solutions that don’t require settings in general. (though this could a be a good exception for that rule?)
- every setting or feature you add requires additional maintenance and testing in the future to make sure it keeps working.
When thinking about it this could be a special text field when setting up a new account under advanced options? but then again it is very advanced and it requires the user to be confident in setting up filter rules on their mailserver or mail provider.