With regards to the To header, this depends on what you munged in the headers you posted. There are addresses in the To header, and I'm not sure if one is yours. There are usually three ways someone can send an email which is delivered to your account:
- To header
- Cc header
- Bcc, which forces blind sending so does not place your address in the normal headers. If you receive a message, your email address will always be in the "envelope" To, which is just what the sending server tells the FastMail receiving server is the destination during the initial SMTP connection protocol. The envelope is normally hidden, but FastMail copies the envelope To into the X-Delivered-to normal header.
So you can see that custom sieve looking at the To header only discovers one out of the three delivery methods shown above. The normal rules are much easier to use, since the "Any recipient's email" condition catches all of the cases described above.
If you use a wildcard alias for your domain, a better way to block certain specific addresses is to make aliases for these addresses which are set to
Reject all mail sent to this address. This causes that address to not exist at the SMTP receiving stage.
Bill