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Slow connection of Outlook 2007 to Exchange 2007 over VPN

We have an Exchange 2007 Server with SP1 running in a domain. The clients (mostly XP Pro)  are running Outlook 2007. They are not using Exchange Casched Mode due to the organizational policy. The performance of clients while they are working in the office where the server is located is OK but for those who connect remotely thru VPN is extremely slow.
The VPN that is implemented is Astaro (ASL version 6) and it is  well tested and is working fine for other softwares with no problem.
Any idea what is causing this?
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Are you running the clients through an exchange set up or are you using the http option?
If you are using HTTP over RPC go into the account options and under exchange proxy settings select 'on slow networks connect over http then by TCP/IP' in outlook 2007.
If you aren't using cached mode then that will be the problem. Unless you have very large amounts of bandwidth you cannot run more than two or three users "live" - as all data is pulled across the internet connection. MAPI is not designed to make live connections across an internet connection.
By large I mean UPLOAD speed measured in 10s of mb. An xDSL line is not enough for more than one or two users.

If you want remote access it is almost mandatory to have cached mode enabled. If you are not allowed to use cached mode then you are going to have to look at your remote access practises and possibly look at using Terminal Services/Citrix instead.

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Thanks for the replies. I am not using http. I use Exchange setup. In one test, I selected "connect to Microsoft Exchange using http." But i observed no noticeable change. I did not configure Exchange Proxy Settings" since I don't have a Proxy Server on my domain. Is that the correct method to use http?
Doesn't really matter whether you are using HTTP or not, you are still going to struggle to use MAPI live.
The "Proxy" is an RPC proxy which is part of Windows that is installed on to the Exchange server.
HTTP is Outlook Anywhere, and really needs to be using SSL.

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Is there any other  light mail client that can connect to MS Exchange 2007 to reduce the connection overhead?
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Thanks for the replies. I guess I will try to use OWA to get a better connection. I have a problem in OWA for emails that are encrypted  and I will post a different question.
I have been having problems with Outlook 2007 connecting to Exchange 2007 using Outlook anywhere (RPC over HTTP). The initial connection speed took almost 30 seconds, and made outlook un-useable during that period.

I found that enabling "On fast networks, connect using HTTP first, then connect using TCP/IP" and setting the manual control connection state to "connect with the network" massively increased connection speed.

Using cached mode and having these options set has made outlook 2007 useable (even on vista).

Want to chime in with Simoncpage's comment, enabling "On fast networks, connect using HTTP first, then connect using TCP/IP"  helped a great deal.

Outlook 2007 on Vista was terribly slow connecting to exchange server -- after launching Outlook, it would take over a minute before an authentication prompt would appear, then another 2-3 minutes for all connections to the exchange server to change to "establish"  and for new mail to appear in my inbox.

After changing that one setting, everything is nearly instantaneous.