Mail forwarded from forms self deletes to trash

Email from a particular form on my Squarespace site self-deletes and sends itself to a trash email folder. A regular email to that email address does not self-delete, only email forwarded from the Squarespace form. It will sit in my inbox for a few seconds of perhaps a few hours, and then suddenly it disappears and ends up in the Trash folder. I have tried everything, as has several different IT folks. There are no rules or filters set anywhere. It is maddening.