Sent messages often do not get saved because of a network or file access error.

When I compose an email, often a pop-up interrupts me to say "Your draft message was not copied to your drafts folder (Drafts) due to network or file access errors. You can retry or save the draft locally to Local Folders/Drafts-[my email address]." It then gives an option to retry (which always fails), save or don't save. It's annoying because it pops up repeatedly when I am writing, and concerning if my messages are not being automatically saved. Does anyone know why this is happening or what I can do about it? Thank you!