Hello!
I had this same issue just a few days ago on my iMac running OS X El Capitan. The problem, for me at least, is that this older version of the Mac operating system isn't compatible modern email message(s) you may have in your Inbox. To fix this, I signed into my Mail provider's online mail interface (since I have iCloud mail, I went to https://www.icloud.com/), and I moved all my email in my Inbox into a new mailbox folder I titled "Older Message [Date Range]". This solved the problem for me.
If this doesn't work for you, or it isn't feasible, you could always use your Mail provider's online mail interface full-time instead of Apple Mail. If you do this, I'd recommend using Google Chrome, since it's the most up-to-date and latest supported browser for OS X El Capitan.
However, if your Mac is new enough, you may be able to upgrade to a newer version of macOS and you won't have to worry about this issue. To see if you can, please respond with what year your Mac is:
Find the model and serial number of your Mac - Apple Support
Jack