macOS Mail has started to delete incoming mail unexpectedly

I have started noticing that email comes into Mail that gets nearly immediately deleted. I notice it either by the system level notification, or looking at the mailbox I see that it comes in. Then in less than one second, the mail disappears. That is, the mail is not anywhere in any mailbox. In particular, it is not in my Today smart mailbox, which should show everything, even spam. So IT IS NOT IN MY SPAM FOLDER. I have re-checked all of my rules, and none of them permanently delete mail, although there are a few that move mail to Trash. I am wondering if this could be caused by the IMAP server in any way. For example, there would have to be some automated action that causes the server to deliver the mail, then delete it. I do have multiple mail services. I cannot tell for sure if the disappearing mail is confined to a specific service, but it seems like it is related to one of them: comcast.net. I have checked the setting on the webmail service for Comcast.net, and I don't see anything that would cause such automated deletions.