redundant mail server

[dbpits]

I'm wondering the way of mail server redundancy.
especially I hope these points below can be accomplished.

-two mail servers (server A and B) serves IMAP service anytime (Active-Active).
-two mail servers (server A and B) are located in different place.(different LAN network)
-two mail servers (server A and B) can be reached from internet in smtp protocol.
-two mail servers (server A and B) can transfer mails to internet in smtp protocol.
-Server B serves WEB mail service.
-All clients usually access server A using IMAP Clients and they can send or recieve mails.
-When clients can't access server A using IMAP Clients , they can access server B using web browser.
(->so they can send/receive mail and read mails which they sent or recieved in past)
-mail data of two mail servers (server A and B) is always same without any human operations.
-mail data of two mail servers (server A and B) are located each locations where each servers are running.
-after system down of server A or server B , automatically mail data of both servers is synchronized without any human operations.

* In these environment, MX servers are located in carrier global network.
They can always transfer internal mail to server A and when they can't transfer mails to server A,
they can transfer mails to server B. they can recieve mails from server A and server B any time.
location between both servers are connected in WAN , so both servers can access each them anytime within LAN network.

anyone know good method to accomplish them above using linux technology?

I'm also researching the way using DFS/IMAP (windows base) but I can't find out the solution yet.

[jledhead]

the problem with redundant imap servers is all mail will always have to exist on both servers simultaneously. you could possibly implement dfs (on windows) or drbd on linux. i will warn though, samba has no idea what to do with dfs.

if you are just handling incoming or outgoing mail and the server isn't a central repo for mail, just an mta, then it would be a little easier. you could use lvs (linux virtual server) to create a cluster of machines.

you can also have a seperate machine for webmail if needed, that shouldn't be a problem.

[dbpits]

hi, jledhead thank you for reply.

current target environmet is that Server A only serve pop3 service and all clients access using pop3 clients.
(mails are not deleted after recieving mails from pop3 clients, so all incoming mail is stored in pop3 server)
In this environment, when Server A is down , all clients can't send/recieve mails
and when client PC is broken, all sent mail is gone.

I also must consider about BCP then mail data and mail application is needed redundantly to be located in each locations
and short duration is required to switch from master environment to slave environment and from slave environment to master environment.
in addition, I hope less human operations when these incidents occures.

Considering things above, I think the best idea is redundant imap servers

In this perspective, I resarched hardware storage replication but they needed to shutdown OS during replication . which means
simultaneous mail data replication is impossible. So I'm considering OS level simultaneous and interactive replication of mail data and
both IMAP application can serve with the replication environment.

I couldn't find out information about redundant imap servers in Internet, so i decide to ask this forum.

but i try to search information about drbd/IMAP.

thank you jledhead.

[thehemi]

Are you planning on running a storage server/appliance
on the backend? SAN or maybe a simple NAS? That will
be necessary if you want the servers to share mail data.
Although I suppose you could have an rsync script which
runs on a very frequent basis to update the backup box
if you only want to use locally mounted disk space.

After you get the servers synchronized, use a DNS entry
to send users to Server A. Use a short TTL value in DNS.
When Server A fails, just update the alias to Server B and
the clients don't need to do anything to failover.

Also remember that, if you're using Outlook, it defaults to
storing the sent items in the local PST file instead of using
the IMAP connection. Don't ask me why.