Free is worth what you pay for it, and sometimes not even that.
Based on the last part of the Rocketmail Wikipedia entry, it looks to be the same service as regular Yahoo mail, with a different domain.
In general and as far as Mail.app (or most any other mail client) is concerned, a mail server is a mail server, and if the mail server is configured to offer the protocols that the mail client supports, it all works.
Most mail servers do offer the common protocols, and are thus compatible across a variety of mail clients.
There are some few cases where (for instance) Microsoft Exchange Server is configured to only offer the Microsoft protocols and thus requires the Microsoft mail clients, or where a particular mail provider - usually a free provider - offers only specific paths into their mail server.