IT Support for Linux Dedicated Server

We have need help about 2 technical issues on our dedicated web server, this is a Linux Centos 6.10 server, we have access to WHM panel, then we login to CPanel for the website(s). 1) The cronjobs defined from Cpanel suddenly stopped. We reboot the server, but no help. I dont know what happened.. It was working during 2 months, then suddenly stopped. You need to fix this and make our crons run again.. 2) Im receiving nearly 100 mails a day from mail delivery system, with subject : mail delivery failed, returning message to the sender, with this body starting; This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: bsaylan@[login to view URL] (generated from root@[login to view URL]) host [login to view URL] [[login to view URL]] SMTP error from remote mail server after MAIL FROM:<root@[login to view URL]> SIZE=2240: 553 sorry, ERROR:your envelope sender domain must have MX or A record ([login to view URL]) (#5.7.1) The system was sending me just alert mails as disk quota warnings etc. Now i receive only this non-sense mails:) I couldnt find how it occurs. You need to stop it, and let warning mails arrive to me. Of course i will give you all login informations which are extremely confidential for us:) There is a webpage running on this server which our customer uses for daily purposes, so we need not to interrupt it, or just we have to warn the customer about the down time. Txs!