Abstract: In some embodiments, a mail verification server can use probabilistic methods to determine whether a user's emails should be throttled based on the recipients to which the user sends emails. The mail verification server can estimate a number of unique recipients to which a user has sent emails in the past, and can estimate whether the rate of which the number of unique recipients increases crosses over a predetermined threshold. The mail verification server can determine that a new user has sent emails to 500 unique recipients, and can track the rate at which the number of unique recipients rises. The mail verification server can then throttle the user's emails if the rate at which the number of unique recipients rises exceeds threshold (e.g., if the number of unique recipients the user has contacted rises by 200% and/or a similar threshold).