Ref T920. This simplifies mail configuration.
The "metamta.domain" option is only used to generate Thread-ID values, and we just need something that looks like a bit like a domain in order to make GMail happy. Just use the install domain. In most cases, this is almost certainly the configured value anyway. In some cases, this may cause a one-time threading break for existing threads; I'll call this out in the changelog.
The "metamta.placeholder-to-recipient" is used to put some null value in "To:" when a mail only has CCs. This is so that if you write a local client mail rule like "when I'm in CC, burn the message in a fire" it works even if all the "to" addresses have elected not to receive the mail. Instead: just send it to an unlikely address at our own domain.
I'll add some additional handling for the possiblity that we may receive this email ourselves in the next change, but it overlaps with T7477.