Simple multi-server installation described in Zimbra-wiki and DNS manipulations. Load balancing is made by making appropriate records in DNS. For example
sn00p@home:/home$ nslookup -q=MX gmail.com
Server: 91.151.241.6
Address: 91.151.241.6#53
Non-authoritative answer:
gmail.com mail exchanger = 5 gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail-smtp-in.l.google.com internet address = 209.85.219.72
alt1.gmail-smtp-in.l.google.com internet address = 209.85.220.36
alt2.gmail-smtp-in.l.google.com internet address = 209.85.216.102
alt3.gmail-smtp-in.l.google.com internet address = 209.85.147.27
alt4.gmail-smtp-in.l.google.com internet address = 209.85.201.114
We have 2 MTA for ~25000 accounts with average traffic 15000 letters (1,5mb each) for a day.