1) Login To Remote Desktop >> "Plesk Services Monitor" [ Right side in the task bar] >> Check the box "Mail Server" >> Click on Restart.
OR
2) Login To Remote Desktop >> Start >> Run >> type " services.msc "
From here, you have restart the Mail Enable [ Mail Server] services such as:
Mail Enable List Connector
Mail Enable Mail Transfer Agent
Mail Enable POP service
Mail Enable PostOffice Connector
Mail Enable SMTP Connector
Right Click on it and press Restart.
