There is a smart function for anti spam called grey listing
The customer can enable or disable it easily.
Grey listing will use a small trick to block ALL mails for 10 minutes the first time.
So if a valid user sends a mail the protector put a 10 minute delay.
And because it is a real mail the real mail server will retry to send the mail to the protector.
When the protector see it resend then it knows it is a real mail server and whitelist the user.
The downside is in the first time they mail it can cause 10 minute delay but only 1 time until they are whitelisted.
Most spam computers are hacked botnes / Windows computers and they can not resend mails.
so by enable grey list it will alone remove about 70-80% of spam.
It is off now and customer can enable if they get more spam just so they are aware there can be small delay in the start.