|After confirming the email settings, scan for malware using Malwarebytes Free. That way we're more confident it isn't a malware problem.|
If Malwarebytes didn't detect a thing, the low disk space message may be legitimate. How much hard disk space does your C-drive have left? You may benefit from running CCleaner:
The settings you check for the Cleaner to remove are really personal preference. For example, I don't remove Windows Size/Location cache and stuff from my anti-malware programs. I also don't remove stuff from Firefox (my web browser of choice) because I prefer using the settings I customized within Firefox. Lastly, don't check Wipe Free Space because it will take forever and is unnecessary for what you're looking to do.
CCleaner also comes with a register cleaner. Although I've never had a problem with CCleaner's registry cleaner and it does ask to backup the entries before deleting them, I should warn you against using any registry cleaner. Deleting the wrong entry could cause problems, so only delete what you're positive won't be a problem, or don't use that feature at all.
Apologies if I don't respond to your reply immediately. I don't check this site daily, but you're welcome to PM me as a reminder.