Plus they should allow as early as possible to connect existing domain names with their service. I wouldn't want to know everybody that I'm sailing under an offshore pirate flag. Primary users would probably come from the grey'er corner of the Internet, so it will most likely lead to some flagging/spam/etc issues.
The reputation is typically assigned to outbound IPs or signing keys, so new domain names wouldn't do much. (Even then, a simple "dig mx your-email-domain.com" would reveal who hosts your email.)
0: https://www.google.com/search?q=megamail