To configure an external mail server to handle mail for your domain, perform the following steps:

1 – Logged in to Plesk.
2 – Go Mail.
3 – Open Mail Settings tab.
4 – Click Activate/Deactivate Services next to the domain you want to manage it.
5 – Select Disable from the menu, and click OK.
6 – Go to the Websites & Domains tab, find the domain name whose DNS settings you want to manage, and click DNS Settings.
7 – Click the hyperlink in the Host column corresponding to the MX resource record.
8 – Type the domain name of your external mail server into the Mail exchange server field. If you are running a remote mail server named ‘mail.myhostname.com’, type ‘mail.myhostname.com’.
9 – Specify the external mail server priority, where 0 is the highest and 50 is the lowest.
10 – Click OK.