Add ability to filter email, only relay matches, or add header

[OJFord]

It might be desirable not to relay anything that SES thinks is spam, for example, or just specifically that fails SPF/DMARC/DKIM.

In order to test that before applying, it'd be convenient to be able to add a header instead, so the recipient can move it to a folder or something in order to test 'this is what I won't receive if I apply this'.

But there may be other uses, so I'm thinking of implementing this as quite a generic 'filter', where 'ignore' and 'add header' are some of potentially more future 'actions' to apply when the filter matches.