I looking for advice on getting better email delivery from our companies direct email relays using Sendmail on Red Hat. We've had some recent problems with grey listing and blocking due to reverse DNS on the sending IP address we lease from an ISP, and perhaps other related things.
I have not setup any reverse DNS with my ISP.
We are our own authoritative DNS, and have our own DNS servers.
We masquerade our outgoing emails as our main domain name. Lets say it is "domain.com".
We have SPF TXT records in our authoritative DNS servers for our "domain.com" with IP addresses (subnets) in them to aid in email delivery. I'm not sure how effect this is, but it doesn't hurt to have it there I guess.
We use Microsoft Office 365 Exchange Online for our email service (not sure if that is relevant). I don't really want to use them for a relay from our internal relay, although that is an option I know about and choose against using a good while ago. My preference is to use direct relays not involving them. I guess if in the end it is a viable option after all is taken into consideration, I can switch to using them as the final relay, I guess. We'll see on that.
The IP address the emails go out on is the first IP address in our ISP block. This is the base IP address for the firewall, and the one the recipient email server sees. Lets say it is "220.127.116.11". It does not reverse DNS correctly to "domain.com". I know I will need to do that at some point in all this of course.
The IP address of our main domain ("domain.com") DNS is different from the first IP address in our ISP block. Let's say it is "18.104.22.168" for "domain.com". Unfortunately, that is the way I have it setup (I'm sure I'm not the only one!). That poses a possible problem in doing the DNS lockups in both directions resulting in a full match. I'm sure there are email servers out there that check that. I'm looking for good (or as best as possible) solutions on that.
We also have multiple WAN connections (3) that outgoing emails might go out on, each taking on the characteristics of the above. I know I will need to do a similar thing on each.