I subscribe to emails from a Google Group. I noticed the other day that I felt like I hadn't gotten any emails about it in a while, so I went onto googlegroups.com and saw that there were many new threads in the group. I went back to my Gmail and searched for the [XYZ] that prefixes every subject line from this group, and lo and behold, there they all were in the trash.
I can now see the emails come in to Airmail and sit in the inbox for a few seconds before the Gmailbot gets a hold on them and deletes them.
I have no filters set up for these emails, and no filters at all with instructions to delete anything. The messages are not being marked as spam; they're simply getting dumped directly into the trash.
Why does Gmail trash these messages automatically?