Why is the Invision Power Board not sending emails?

Question

PHP

Good afternoon! There is a forum on Invision Power Board 3.4.6. All of a sudden it stopped sending e-mail to both administrators and users. When testing sending mail, "Support - diagnostics - checking Email" writes "Email sent successfully" - but nothing comes.
It is interesting that if you write a test script that uses the classEmail mail class from the \ipsKernel\classEmail.php file, then test letters go. Those. sending from the IPB itself does not work, although in the end all the necessary parameters ($to, $subject, $message, $rfc_headers) are transferred to the mail() function and have quite correct values, which I looked through echo / var_dump. And mail() returns true despite the fact that the letter does not reach any recipients.
In what direction to dig?
PS. Hosting sweb, technical support says nothing intelligible.

Answer 1

Letters are not sent from [email protected] by chance?

Answer 2

See server logs. mail queue etc.

Answer 3

Guys, this new DMARC policy from mail.ru is a dirty trick though)