We are planning to use zimbra for more than 10k users out of which few users only receive mail from internet and almost 95% of the users should not able to send or receive mails from/to outside. Even the remaining 5% users would not receive mails from all of the domains and would want to receive mail from one domain i.e. gmail.com.
I am more concern about server sizing and I am pretty sure one server would not take this much load hence planning for below setup and need all your expert advice.
one Master LDAP server
Two Mailbox servers
One incoming server
one outgoing server
What else you guys think I should make sure before start implementing?