You could do this with postfix.  Setup the "MAIN" server as a postfix 
mail gateway, and use your local server as the imap server.  The 
messages will arrive at the "MAIN" server that your MX record points to 
(where they could be spam/virus filtered) and then would be immediately 
passed to the local network server (where users could access them via 
pop/imap).  I don't know if this is what you are looking for as users 
would need access to the local network to even read their mail, and you 
would only have one persistent copy of the hosted mail (instead of the 
office being a secondary backup), but it would help to increase speed.

--Kirk

Claudinei Matos said the following:
> Yeah, I saw that the problem is ocurring 'cause all the users share
> the same ip which one is the ip of the internet gateway.
> Anyway, with the new limit of connections everybody can retrieve mail
> but some users are annoying me about the speed to access the server.
> What I want is that the messages that arrive at my MAIN (the one at
> the hoster)  server could be redirected to my proxy/backup server (the
> one at my local network) or if it could be, that the proxy/backup
> connect to the main server to catch all the accounts messages.
> I know I culd just use my local server as my MAIN MX but the problem
> is that this server is connected to internet by a adsl not confiable
> and with dynamic ip link. So I was thinking if I could setup this
> local server as my primary MX with his dyndns hostname and put the
> "old main" server as the backup MX, just in the case adsl link get
> down.
> I've also got a quick look at
> http://www.courier-mta.org/imap/?README.proxy.html and I saw that I
> can setup a proxy server to my users, but I don't know if that could
> be a solution too.
> 
> Any advices would be appreciated.
> 
> Tks,
> 
> Claudinei Matos
> 
> On 5/16/05, Jonathan Nichols <jnichols@...> wrote:
> 
>>Claudinei Matos wrote:
>>
>>>I was looking at this parameter when I got your mail. Actually it was
>>>specified a limit of 4 connections. I tried to setup to 0 ( I mean it
>>>could be no limit) but doesn't work, so for a test I did setup it to
>>>30 connections and did some tests, and I saw that each folder that I
>>>try to open in thunderbird open a new connection at the server. So if
>>>I have 10 users trying to see his email and 5 of they try to open 2
>>>folders at the same time I will have 20 connections on the server. Is
>>>that correct?
>>
>>Pretty much, yeah. but, it's per IP. I have mine set to 180 and haven't
>>had any problems at all. :)
>>
>>--
>>gentoo-user@g.o mailing list
>>
>>
> 
> 
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