Using SMTP Relay on a Exim Server in VPS for Multiple Domains

To expound on what Halfgaar has said:

You can operate multiple ourgoing SMTP servers and what you probably want to do is to set up an SPF record in your public DNS namespace that identifies all SMTP servers that are authorized to send email for your domain. Also set up appropriate A and PTR records for any SMTP server sending email for your domain. This is not a guarantess that you won't be blacklisted, but it will go along way to making sure you're not. Make sure that any MX records set up for the domain only point to the third party email provider as you don't want any inbound email going to the onsite SMTP server. Also make sure that any email sent from the onsite SMTP server goes out with reply to addresses that are valid so that replies are sent to legitimate recipients at the third party email provider.

RDNS is irrelevant - it only checks that the IP address maps back to the domain name use.

So: x.x.x.x maps to smtp.hoster.domain then the MS must name smtp.hoster.domain. As an MX always contains the IN A record, no CNAME, that simply means you put in the hoster's / shared domain name there as FQDN that properly maps.

Basically, I run a SMALL hosting operation. Just for myself and some poeple.

THe SMTP incoming / outcoming email gateway is:

smtpgateway.x.y

In my own domain (company.domain) I Have an MX record that reads "smtpgatway.x.y". Finished - RDNS resolves properly.

So, basically, you need ONE ip for unlimited domains, the rest is something your smtp server does (i.e. knowing how to handle them, put them to separate mailboxes etc.).