Here are the two largest mail servers I man­age. They han­dle over 20K user­s, with full mail ser­vice.

[root@nmail ~]# uptime
 11:02:29 up 217 days,  3:09,  3 users,  load average: 1.54, 5.79, 7.40

[root@correo1 ~]# uptime
 11:02:25 up 227 days, 21:58,  1 user,  load average: 6.78, 4.61, 3.90

Ok, the load is a lit­tle high, but I know the rea­son and it's not a big prob­lem.