The Emails works in two ways, one is the Incoming emails and another is the Outgoing emails. The Outgoing emails are sent from one domain to another domain, the Outgoing messages have the limitation to send from the domain. A default value can be set for the limit of on outgoing email messages from a domain by using the mailserver utility. This utility can manage Plesk mail server settings. The following command is used to set the maximum number of outgoing messages from a domain using Plesk.
mailserver --set-outgoing-messages-domain-limit10