What should be the rDNS PTR records for a self-hosted email server with a website domain hosted elsewhere?

We have a self-hosted mail server at our business's single, static IP address (mail.companyA.com). Our website is hosted by another company at a different IP address (companyA.com). Our emails use the @companyA.com domain.

Sometimes we send emails to certain clients that get through fine, and sometimes we get bouncebacks from the same client indicating we are blocked. Short-story: I am concerned that a reverse DNS lookup done by some companies with stricter requirements on emails is getting us in trouble depending on the rDNS lookup result that appears first.

MXToolbox reports that a rDNS lookup on our static IP address (from which we sent our emails) has two PTR records: One to mail.companyA.com and the other to companyA.com

Should I request our ISP to remove the PTR record to companyA.com, leaving only mail.companyA.com - or will this create problems?

The issue here is usually related to Forward Confirmed reverse DNS (FCrDNS) checks, which are used as proof-of-work tests by some mail servers.

A receiving server may perform a check using the sending server IP address PTR record and confirm that an A record exists that matches the PTR record. Some mail servers even go further and verify a 200 response.

If your mail server is located at mail.example.com, then the PTR for the IP address of the server should be mail.example.com.

Note that an IP address can have only one PTR record.

A more technical explanation that includes other reasons to configure FCrDNS are included in RFC 1912.