I have run a search and did not find this specific issue discussed. I have been using AquaMail pro for a couple of years now and it is by far the best email app I have found.
On my Galaxy tab2 7.0 (android 4.2.2) it works flawlessly, however on my Galaxy S4, (android 4.4.2) AquaMail is unable to save attachments or write backup to my extSD. I am unsure of whether this was an issue before the recent update to 4.4.2 or not. I spoke to the cellular folks and they have no idea, I am wondering if there is a workaround short of manually moving the files to the destination I choose from the file browser?
I have cleared cache and data, uninstalled and re-installed, all with no change