php mail function fires every other time?

Question

PHP

Mail does not work on the server, I check the standard mail function

if(mail("[email protected]", "My Subject", "Line 1\nLine 2\nLine 3")) echo "message send";
else echo "message not send";

I launch a script, works, the letter really comes.
The second time doesn't work. Not immediately, not in a minute.
Then at some point I try again - The mail has gone. And again it doesn't work.
Centos, php 7.2, vps
Where can I see what?
In the logs (if they are):

/var/log/mail.log
Jun 21 14:21:54 example dovecot: master: Warning: Killed with signal 15 (by pid=12621 uid=0 code=kill)
Jun 21 14:21:55 example dovecot: auth: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Jun 21 14:21:55 example dovecot: anvil: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)
Jun 21 14:21:55 example dovecot: log: Warning: Killed with signal 15 (by pid=1 uid=0 code=kill)

Answer 1

mailq
postsuer -d ALL

Answer 2

The kagbe panel tells you that it cannot restart the dovecot service. Here is a similar problem Ubuntu server 14.04 dovecot error (core dumps disable)? and you can also look here https://forums.cpanel.net/threads/dovecot-master-w...
Most likely there is a problem with the certificate.