How to limits the number of outgoing email messages for a domain in Plesk
To set up limits on the number of outgoing email messages for a domain, perform the following steps:
1 – Logged in to Plesk.
2 – Go Mail.
3 – Open Mail Settings tab.
4 – Select the domain you want to manage it.
5 – Click Limit Outgoing Messages on the toolbar.
6 – Edit the Limit on outgoing messages field.
7 – Click OK.