Two mail server installs (for two domains) on Windows VPS with one IP?

I'm referring to a VPS that's running software for mail services that's supplied by the VPS host. The VPS has 1 IP address and the mail software license supports only 1 domain.

I'm now adding a second domain to the VPS on the same IP address. There's a quite large increase in cost to license the pre-installed mail software for 2 or more domains. For that reason I'd like to configure a different brand of mail server software onto the VPS to support the second domain (I'm not married to the pre-installed mail software).

My foremost constraint is that I really don't want to disrupt the pre-existing mail software which right now is installed on the VPS and successfully supporting the first domain. There's no reason to break or disrupt the original domain, and TBH a full replacement of the supplied mail software seems likely (to me) that it will break or at least disrupt the original domain's mail.

Thinking toward the example in which IIS can be configured to support more than one domain on a single IP, I wonder if there's a technique "out there" that will allow two brands of mail software to be configured onto the VPS, with 1 IP address, where my 2nd domain is supported by the new mail software without disrupting the original domain/mail software. I understand this might be wishful thinking, but I'm asking in case there is a solution that I'm simply not aware of.

Thanks

While it would be possible to have two domains sending mail from two separate mail servers via a single IP address, it is fairly involved to have two mail servers both receiving mail from a single IP address. The thing is that a mail server, to receive mail, has to be continuously listening on port 25, and that cannot be shared. IIS can serve multiple sites because there is only one program, IIS, listening for connections; similarly a mail server can listen for multiple domains because, again, there is only one program listening for connections.

The only way I could see to do this is to move your existing mail server to a different listen port for incoming mail, then have all mail go to your new mail server; and have that mail server configured to forward everything for the existing domain on to the non-standard port where the existing mail server is listening. Whether that would be as disruptive as replacing the existing mail server, I cannot say. If adding the new domain to your existing server is cost prohibitive, setting it up to forward the new domain to a new mail server likely would be at least some fraction of the same cost.

There isn't. They both need to listen on the same ports (25 for smtp).

You can have one webserver serve multiple domains, but you can't run multiple web servers.