When the play service is being stopped or when it is crashing a bunch of email processes that were still running are being listed in the logs. Those most likely never completed.
See extract of the logs below:
[info] org.w3.vs.actor.EmailServiceWorker - Stopped child email worker after attempts: 1 - john*******@gmail.com - W3C Validator Suite registration confirmation
[info] org.w3.vs.actor.EmailServiceWorker - Stopped child email worker after attempts: 1 - paul*******@gmail.com - W3C Validator Suite password reset
...
When the play service is being stopped or when it is crashing a bunch of email processes that were still running are being listed in the logs. Those most likely never completed. See extract of the logs below: