To set up limits on the number of outgoing email messages for a domain, perform the following steps:
1 - Logged in to Plesk, https://your domain name:8443.
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.