There are a few things you are going to want to consider in this. First, you really need to define what you want the mail system to be. If your requirements are simply POP/IMAP, then you can go with a variety of vendors or, if you are big on building it yourself, some opensource offerings.
However, with a million accounts, you aren't going to want to revisit all the outlook clients to point to POP/IMAP. For a direct exchange replacement you might want to talk with communigate (formerly stalker).
You will also want to consider looking at the mail gateways for inbound and outbound traffic. While opensource is a great thing, managing the infrastructure for a system this large on opensource can be a bit of a pain. So, I would look at the commercial vendors out there like IronPort as they provide all the filtering and traffic management through a single interface.
The vendors will provide all the design assistance you will need, so there really isn't a need to bring in a consultant.
So, my design would be to have an MTA layer facing the net (potentially dedicated inbound and outbound MTAs), the mailstore layer in the protected net, an LDAP master server and a couple of replicas for the mail systems to hit. I know IronPort and Communigate would do this well.
While I am a big fan of open source, for a system this large you really want support and companies backing up the whole thing...
LEt the flames begin
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There are a few things you are going to want to consider in this. First, you really need to define what you want the mail system to be. If your requirements are simply POP/IMAP, then you can go with a variety of vendors or, if you are big on building it yourself, some opensource offerings. However, with a million accounts, you aren't going to want to revisit all the outlook clients to point to POP/IMAP. For a direct exchange replacement you might want to talk with communigate (formerly stalker). You will also want to consider looking at the mail gateways for inbound and outbound traffic. While opensource is a great thing, managing the infrastructure for a system this large on opensource can be a bit of a pain. So, I would look at the commercial vendors out there like IronPort as they provide all the filtering and traffic management through a single interface. The vendors will provide all the design assistance you will need, so there really isn't a need to bring in a consultant. So, my design would be to have an MTA layer facing the net (potentially dedicated inbound and outbound MTAs), the mailstore layer in the protected net, an LDAP master server and a couple of replicas for the mail systems to hit. I know IronPort and Communigate would do this well. While I am a big fan of open source, for a system this large you really want support and companies backing up the whole thing... LEt the flames begin