Set up the work of sending letters to customers through the admin panel of the site on Webasyst Shop-Script 3.

Letters do not reach customers, they are returned back.

Information from hosting support:
Checked the information on the mail server.
At the moment, letters are sent through the site by the mail() function.
It was also noticed that the messages were rejected into spam by the recipients server. Configure authorized sending of emails and configure a DKIM signature to prevent this situation.
(Instructions included)
You can manipulate emails within your site using PHP.

The response of a specialist familiar with the engine is very desirable.