You want to use Aloha load balancer in front of a mail platform, relying on HAProxy to achieve load-balancing.
You mail platform delivers IMAP acts as a SMTP relay for your users.
You want your users to use the same server for both protocols in order to take advantage of different caches level (FS cache, authentication, etc…)
The HAProxy configuration below explain how to force the SMTP relay for IMAP connected users.
Users relaying mail through SMTP will be connected to any server in the farm using round robin.
frontend ft_smtp bind :25 mode tcp default_backend bk_smtp frontend ft_imap bind :143 mode tcp default_backend bk_imap backend bk_smtp mode tcp balance roundrobin stick match src table bk_imap server s1 192.168.1.1:25 check server s2 192.168.1.2:25 check backend bk_imap mode tcp # imap connections are usually long balance leastconn stick store-request src stick-table type ip size 200k expire 30m server s1 192.168.1.1:143 check server s2 192.168.1.2:143 check