I was having a niggling issue when bouncing back and forth between the macOS Mail app and the web interface of Gmail. If I started composing an email in Mail I found that when I went to view the inbox via the Gmail web interface I was presented with a litany of (mostly) empty Draft messages. Annoying.

There was a simple solution which I found in Mail's Account Preferences. It is thus:

Fire up Mail
Go to Preferences...
Select Accounts tab
Select Gmail IMAP account
Select Mailbox Behaviors
Uncheck "Store draft messages on the server"
Save
Life and your inbox are in harmony once again

The account preferences for draft messages should look like the following: