Email Client and Server Test Process and Procedures???

Hello,
How are you doing today?
I am getting ready to setup an email server (Postfix) and client (Kmail).
I’d like to know what are the “best” test procedures and processes for handling 300 local clients (office) and 25 remote clients. I’d like to know, how long should I run (test) the client/server configuration before putting it into production???

If I have missed on ANY important question, PLEASE let me know.

I look forward to hearing from you…

Thank you,