We have a single domain solution (also an additional alias domain to deal with legacy users brought in from another ISP) in place using iRedAdmin Pro LDAP serving about 5,000 users. It likely doesn't make much of a difference, but we query an external LDAP server. We are also running a separate non pro server using the same LDAP server to manage all of our Radius users (no mail, authentication only) - about 500 PPPOE and 5,000 or so Option 82 devices. We are an ISP and we are using this for all of our customer mail.
Centos 7, running on a vmware VM with 5 CPU and 4GB RAM. It's been running for a bit over a year and a half, with no issues whatsoever (other than a few configuration and performance tweaks).
I can't recommend this solution highly enough, and the support is first class.
Was wondering if anyone has done a large scale deployment (~8,000 users/1 domain) using iRedMail?
Thanks,
David