PHP mail form stopped working after updating Plesk to version 18.0.38

Symptoms

• A custom mail script stopped working after updating Plesk to version 18.0.38 or later.
• The following error is output to /var/log/maillog:

  CONFIG_TEXT: Sep 7 22:05:51 test_vm plesk-sendmail[29901]: S29901: Unable to prepare arguments for mail handlers.

Cause

A newline is missing between email header and body. Before 18.0.38 such emails were still sent via OS sendmail as a fallback, but due to logging change in 18.0.38 it is now considered a critical error and sending is stopped.

Resolution

Modify website code so email will contain a newline between headers and body.