Rooting has nothing to do with it. It's a permissions issue.
Google made the change in 4.4, but not all manufacturers started encoforcing it back then (I've got a Motorola that still allows write access to the microSd, on Android 6).
There are no plans at this time to implement the necessary workaround in Aqua Mail:
- when Google brought this ability back in 5.0 (if my memory serves), they made it so that accessing files on the microSd specifically requires all new functions (before, it was "a file is a file, no matter where").
- in addition, there is a "thing" that the user would need to do before writing is possible, and it's quite non-trivial ("when you tap this button, you'll see a screen with X and Y, please select X and then tap Y").
So this part of the app would need to be rewritten. Given that Aqua Mail is not a file manager, this is not a priority (when weighted against other work).
Please note that since version 1.6.2.9, Aqua Mail is able to "cache" attachments on the "real" microSd, starting with Android 6. "Cached" attachments are typically the bigger part of the app's storage requirements.
App settings -> look and feel -> Folder for caching attachments -> On the memory card