I believe that will be more useful to have an outgoing message limit per user beside the per domain limit that already exists, on your competence already exists since the last 2 versions and it's very helpful to find spammers.
Mail sending limits per email account (rather than per domain)
In a discussion with a cPanel Partner NOC, a suggestion came up that cPanel integrate per e-mail account mail sending limits, similar to the existing limits that can be configured in Tweak Settings that limit how many emails a domain can send per hour.
One goal of such functionality would be to avoid punishing other email users on a domain simply because 1 user decided to be abusive by means of sending bulk email.
Would you find this feature useful? Do you have any suggestions on how we should implement this feature?