Lately, my phone is constantly giving me messages about running out of space, so I’ve been working on moving as much as possible over to my external SD card (which is ginormous and has plenty of space). In K9, I have all of my email accounts set to use the external SD for storage, and yet the app is still using well over 300mb on my phone’s internal storage. What’s going wrong? Since all these emails are merely synced copies of what’s on the server, they seem like ideal candidates to be stored on the external card; if it dies, I won’t really have lost anything, except perhaps the time & bandwidth to redownload them once a new card is installed.
My specs:
K9 5.600
Samsung Galaxy S5
Android 6.0.1
K-9 Mail doesn’t support storing messages on external storage media. The setting is left over from a time when Android storage was simpler and there was only internal and external storage. In newer Android versions both of what was previously called internal and external storage are located on the storage built into the device. What users perceive as external storage these days is “external storage 2” from a developer’s perspective. K-9 Mail doesn’t support that and we have no plans to do so. However, we removed the “Storage location” setting in 5.7xx beta versions because it’s not doing anything useful.
Very disappointing. 
It sure would free up a lot of space on my phone if it worked!
Same here. 
Having many accounts, my storage on my tablet was full very soon.