My university's #IT claims that the .host TLD is "potentially suspicious", and therefore all incoming email from any .host domain should be ignored without even notifying the receiver.
If you hear from someone who doesn't receive their confirmation mail from a #mastodon instance hosted on masto.host, this may be the reason (even if the instance has a different domain, mails will originate from masto.host). Such (IMO inacceptable!) policies are often shared in the IT community.