With many people using reverse dns to validate email (such as aol) how important is it to get this right and can this be supported on a shared server with multiple domains?

reverse dns is checked in respect to your mail server, its nothing to do with your email address. If you have a server smtp.mydomain.com with ip 1.2.3.4 then the reverse dns for 1.2.3.4 must be set to smtp.mydomain.com or your email will be blocked by all sensible mail systems. your mail server should only have one hostname regardless of who it is sending email for.

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