Question
How to change the mail server's domain name?
Answer
The process differs depending on whether the domain uses Plesk DNS or an external DNS server.
Plesk DNS
External DNS
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Go to Domains > example.com > Hosting & DNS > DNS 
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Look for the MX record type and open it 
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Set Mail exchange server to the desired domain Note: The MX record value must have a corresponding A record. 
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Connect to the server via SSH 
- 
Find the domain's DNS provider # dig +short example.com NS 
 ns2.he.net.
 ns5.he.net.
 ns4.he.net.
 ns3.he.net.
 ns1.he.net.
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Log in to the DNS provider's settings page 
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Set the MX record to the desired domain