For most transmissions of digital information (even those here on earth) there’s a concept of a “checksum”. Basically at the end of every message, there’s a special number, and you can do some math on the rest of the message to get that same number. If anything happened to change or damage the message in transit, the math doesn’t work out and so the checksum fails.

I would assume Voyager works in a similar way so every time it receives a message it will compute the checksum and see whether it matches

You should definitely set up a DMARC record to prevent other people from using your email domain to send spam. If you don’t have DMARC configured, other email servers will give any senders the benefit of the doubt and accept mail that claims to be from your domain.

You can just set the DMARC record to reject 100% of unverified mail and call it a day. Since you aren’t sending anything it won’t affect you.

These ads only appear in the “promotions” section of Gmail, the section that is by definition for advertising emails. It’s not great, but this is the least intrusive place to put ads.