Ignore my answer, I misunderstood your question. This is not expected. File a bug with reproduction instructions.

I was unable to reproduce on secure.phabricator.com, so I'm going to investigate a little further.

Edit: I updated my Phabricator install, and restarted everything (daemons/nginx/php-fpm), and the original issue I was having seems to have gone away. At the same time, a ton of email seemed to get delivered from actions that were taken much earlier in the day (people accepting revisions, emails for inline comments, etc). It's unclear why this email was "stuck", but isn't now. The only thing I can say is that when at least one of these inline comments was created earlier in the day, I checked my Inbox and Outbox in the Mail application and saw that no email had been generated for my comment.

In any case, my immediate issue is resolved, but I'll keep an eye out for email that seems to be "stuck" and see if I can figure out a way to reproduce. Thanks for your answer/comment!