Before installing your proxies, you need to consider the following guidelines. See Chapter "Configuring SIMS as a Proxy Message Store server," in the Sun Internet Mail Server 3.5 Administrator's Guide for different proxy scenarios, models, and configurations.The proxy servers are for two real solutions: one is for security by putting the proxy on the firewall and the backend mail server on the other side. The other is for horizontal scalability to use multiple backend servers and multiple proxies.
Typically, each user has a well-defined backend mail server but accesses a random proxy via DNS round-robin. The advantage compared with just multiple servers without proxies is that all users access one single server name rather than each user having to configure their own mailhost.
Once the client is authenticated on the proxy, it will contact the real mailhost and redo the login using the same username and password. After login, however, the proxy switches to a passive relay mode in which everything that comes from the client is passed onto the backend mailhost and vice-versa without any interruption on the part of the proxy. Overly large machines should not be required for the proxies.
When configuring for number of users, you must look again at the total concurrent users planned for, the total number of users, the percentages of POP3 and IMAP4 that will be concurrent at peak times, and the size of the users' mailboxes. From this you can estimate the needs of the backend mail server or servers. The number of proxy servers that you need depends upon the model you choose. That is, corporate versus ISP.
Note - Be sure that you install pure proxy on a clean system where no SIMS files are installed.