So, we have actually blocked new registrations for users that is using gmail.com. This is because, 99% of all spam accounts uses gmail.com 80% gmail.com signups is spam.
Just did a quick test - I enabled registration for gmail.com email-addresses. And within 10 minutes had three spamaccounts been registered and started to spew spam.
@selea funny, i recently had a discussion about self hosting mail here on fedi. i made the point that gmail is a big spammer themselves. thanks for the additional data point ;)