Update: 8/31/2015, I take it all back. It was only a temporary fix. My new diagnosis is something has corrupted my account or the mail app is mishandling things. Will update if I find something that works.
I’ve been pretty aggravated lately with a recurring error with Windows 10 built-in mail client with my work e-mail account.
Luckily, this fix worked for me to clear it up.