Hi,
Do you have another spare server with you at the moment? It would be great to have a dedicated server for it. But since you say you only have three domains using this mail server then a VM will do just fine. A dedicated server is always good though. . . In any case, I can handle this project for you either way. .. . .
... In fact, I had exactly a similar job for a client in US who had her mail server setup on a FreeBSD server. . . Unfortunately she has developed a Multiple Sclerosis disease and we have lost contact. I hope she's doing fine. . .
Any way, I have had my own hosting business where I have setup/maintained numerous mail servers and I have done it for many clients. I would love to do the same for you. :) ..
Let me know if you have any questions or concerns, . . . I hope to hear back from you soon..
Cheers...
Kashif