If you have ISPmanager, you need to add your VPS email into site settings for correct emailing by scripts.
To do this, create a mailbox (if you haven't created it yet) and specify it as E-mail of site administrator in the site settings.
Everything! Mail should work without problems.
Read more information about setting up mailboxes in ISPmanager5 [here] (https://fornex.com/help/mail-isp5/).