I have a Linode VPS and am happy to fire up another VPS to handle email exclusively. In fact, I already did and installed Mail in a Box as mail.mydomain.com. I have to say (being a developer with enough Linux knowledge to handle hosting and simple stuff) that nameserver setup is completely confusing. I should stop using Linode nameservers? If so, I should set up glue records and place that into NameCheap domain specification instead? And then set up www CNAME to point to original website server? Very confusing... Why not having a section in the guide titled something along: "Adding Mail-in-a-Box as additional server to your existing website"? I am not sure if that defeats security/reliability, as External DNS section message is kind of scary:

"Although your box is configured to serve its own DNS, it is possible to host your DNS elsewhere. We do not recommend this.

If you do so, you are responsible for keeping your DNS entries up to date. In particular DNSSEC entries must be re-signed periodically. Do not set a DS record at your registrar or publish DNSSEC entries in your DNS zones if you do not intend to keep them up to date."