Never-ending notification emails

I’ve run into a strange situation after setting up email for a new instance. I’m on the v13 beta, and I am using gsuite and my email service provider.

Short version: I’m getting the exact same email notification resent to me every 4 minutes. Even if I Suspend Sending from the Email Queue, the emails keep coming.

Longer version:
The notification being sent repeatedly is of a very specific type. Specifically:

  1. Using a company-wide email address ([email protected]), a user sends an email directly from a document to [email protected]. Effect: The email sends as intended and is appended as a communication to the bottom of the document.
  2. Next, [email protected] receives the email and replies. Effect: the reply email is received and gets listed at the bottom of the document, as intended.
  3. Then (and this is where things go haywire), using my default sending address, ERPNext forwards the email received from [email protected] to the user’s individual email address ([email protected]). This notification email keeps getting sent over and over, and short of deleting the email addresses involved I’ve not found a way to stop it.

I’m struggling to troubleshoot this, as I can’t figure out where in the code base these third notification emails get sent. I’m hoping to get a bit more information from anyone familiar before posting an issue.

Thoughts or ideas are greatly appreciated!

For posterity:

I was able to stop the onslaught by changing Email sync option from “ALL” to “UNSEEN”. I don’t really understand how this setting works. I was hoping to use the same email address in both ERPNext and a traditional email client, but it seems that may not work well in practice.