Do your wife and you use an anti-virus product to scan outgoing and incoming mail ? If yes, then try this: suggest you open the anti-virus product and select to not scan incoming and outgoing mail.

Not thought of that - however both email accounts can be used on more than one device, but it is the same computer that is the problem. I'll try an alternative method. Anti-virus is Kaspersky btw.

Try changing the filter to function after classification. What is probably happening is the message is being forwarded before the download fully completes. after classification delays the filter execution.

I have gone for copy rather than forward.