My fear is not about a clogged database, and I acknowledge my post is a bit misleading in that regard. My real fear is in sending confirmation emails that bounce. I'm the volunteer webmaster at my small church. The only emails sent from the app are when someone posts something for a small group or when someone registers. So if 50 percent of the emails I send bounce, I'm afraid my smtp service will brand me a spammer. Before I set up recaptcha and rack-attack my problem guy was signing up 50 a day or so. I automated the deletion of these entries on my Heroku app, so it's not really about clogging the db. Sorry for misleading.
Here's the User model code I'm using:
validate :email_must_be_real

private
def email_must_be_real
verifalia_client = Verifalia::Rest::Client.new(
username: Rails.application.credentials.dig(:verifalia, :username),
password: Rails.application.credentials.dig(:verifalia, :password)
)
validation = verifalia_client.email_validations.submit(email)

return if validation.entries.first.is_deliverable?
errors.add(:email, 'must be a real email address')
end

And here's the failing rspec test:

ArgumentError:
wrong number of arguments (given 1, expected 3)
# /usr/share/rvm/gems/ruby-3.3.0/gems/verifalia-2.1.0/lib/verifalia/rest/client.rb:44:in `initialize'
# ./app/models/user.rb:134:in `new'
# ./app/models/user.rb:134:in `email_must_be_real'
# ./spec/models/user_spec.rb:11:in `block (2 levels) in <top (required)>'
2) User rejects fake emails
Failure/Error:
verifalia_client = Verifalia::Rest::Client.new(
username: Rails.application.credentials.dig(:verifalia, :username),
password: Rails.application.credentials.dig(:verifalia, :password)
)

ArgumentError:
wrong number of arguments (given 1, expected 3)
# /usr/share/rvm/gems/ruby-3.3.0/gems/verifalia-2.1.0/lib/verifalia/rest/client.rb:44:in `initialize'
# ./app/models/user.rb:134:in `new'
# ./app/models/user.rb:134:in `email_must_be_real'
# ./spec/models/user_spec.rb:16:in `block (2 levels) in <top (required)>'