[shellwizard]

I think he refers to mail headers. Those are normally hidden away from you, at least on a PC browser you can see them:

1. Open the specific email.

2. Click the three vertical dots (More options icon) next to the "Reply" button.

3. Select "Show original" from the dropdown menu.

[no-name-here]

Mail headers also include to:, from:, subject:, etc, as well as more obscure items too, which is why I think OP commenter meant something very different than “cannot filter on headers”.

Also, more items that might help OP (as I can’t edit parent comment) - they mentioned wanting to use wildcards on to: field. Those header fields do allow specifying just part of the header, like just the domain, or one part of the to address. (But those match at word boundaries and I’m not aware of being able to match sub parts of words or more complex items.)

Regardless, I don’t think I’d call this a “major” feature gap - maybe minor or more of a niche feature.