I have a webserver that host multiple domains. emails get sent from this host to members of the various domains with their domain in the email address: joe@thissdomain.com, ralph@thatdomain.org, etc. I have MX records for these domains and sendmail is configured with these domains in /etc/mail/local-host-names.

The problem is, I need to specify reverse DNSes for these hosts or some recipients (outlook.com) reject the messages and I end up listed in spamhaus.

My ISP sets up rdns and in the past I've only give one domain to the ISP. How do I get this straightened out? Do I have to give the ISP each domain so they can create individual rdns lookups?