We manage a vps that works perfectly with all your options but the delivery of emails takes at least 2 hours.
In other words, if you send an email from an external mailbox to a mailbox in a domain hosted on the server, it takes about two hours to arrive.
Gray list is disabled and has always been
If we run cat / etc / resolv.conf, the Google DNS is configured
There is no bounce code, emails arrive but two hours later, there is no waiting queue for publication.
We do not use any relays or anything of the sort, the mail is processed by default by cpanel exim.
Can someone help us?