Website contact forms just stopped working

I saw couple days ago that all forms in multiple websites just stopped working. I have several Wordpress websites on one server, Im using latest DirectAdmin, all updates installed and it just not sending emails from forms.

When I try to send message from form I get an error: "There was an error trying to send your message. Please try again later."
It is not from wordpress fault, it just stopped on all websites in different WP versions.

Same case when I try to send simple form in clean php script.

It looks like phpmail? function stopped or was blocked. I wasn't made any configuration changes / not touched server in that time, so I assume it is not my changes.

Do you have any ideas how to make it work again?

Please help me fix this.