Issue Symptoms

• Unable to send email via web form that is written on php.
• Nothing can be found in /var/log/maillog
• Outgoing anti-spam is enabled on the server.

Cause

Allow using Sendmail for scripts and users on this subscription option is disabled on subscription.

Resolution

Make sure that Sendmail utility is allowed to use:

Plesk > Tools & Settings > Mail Server Settings 

Plesk > Subscriptions > example.com > Change Outgoing Mail Limits