Question: I am familiar with archiving messages on the client side to .pst's... Are there good methods of archiving messages from multiple mailboxes on the server side?

Answer: 
Exmerge is not supported on Exchange 2007 (although it can be made to work). As of Exchange 2007 SP1 there is a new cmdlet, Export-Mailbox, that replaces the exmerge functionality allowing you to archive multiple mailboxes to PSTs.

This cmdlet also works with PSTs up to 32GB in size. However bizarrely it only works on 32bit Windows, since Exchange 2007 is only supported on 64bit Windows you need to set up a 32bit machine to do the archiving (Thanks MS).

The whole process is detailed [here](http://www.exchangeinbox.com/article.aspx?i=102).
For exchange 2003 and below the best tool I've found is called [exmerge](http://www.microsoft.com/downloads/details.aspx?familyid=429163EC-DCDF-47DC-96DA-1C12D67327D5&displaylang=en) and is available from Microsoft. It allows you to pull or push users data into psts on the server side and filter by date, folder, type etc. You can also set it to batch export a list of users. Its very easy to use and very handy. 

I use it for archiving of old users, moving users between servers and backup of important users so I can have quick access to their backups. (As opposed to restoring parts of the exchange database to a different data store. Exmerge is just easier.)

PST files shouldn't ever really be bigger then 1 or 2 gigs, I've run into corruption. So I usually filter by date (an archive per year) if it's a particularity large mailbox. It should also be noted that with exchange 2003 mailboxes can now support Unicode and exmerge does not.

From the download page.

> 
> Use the Mailbox Merge Program to extract data from mailboxes on a Microsoft Exchange Server and then merge this data into mailboxes on another Microsoft Exchange Server. The program copies data from the source server into Personal Folders (.PST files) and then merges the data, in the Personal Folders, into mailboxes on the destination server. The ability to merge data to and from an Exchange Server makes this program an invaluable tool with a variety of uses- especially during disaster recovery. The program can also replace existing data instead of merging new data if specified by the Administrator.
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Exmerge is not supported on Exchange 2007 (although it can be made to work). As of Exchange 2007 SP1 there is a new cmdlet, Export-Mailbox, that replaces the exmerge functionality allowing you to archive multiple mailboxes to PSTs.

This cmdlet also works with PSTs up to 32GB in size. However bizarrely it only works on 32bit Windows, since Exchange 2007 is only supported on 64bit Windows you need to set up a 32bit machine to do the archiving (Thanks MS).

The whole process is detailed [here](http://www.exchangeinbox.com/article.aspx?i=102).

[GFI MailArchiver](http://www.gfi.com/mailarchiver/) is a 3rd party tool that has outlook integration. There are few products that fill this role.

Google's [Postini](http://www.google.com/postini/) will archive your mail, make it searchable, and store it in the "cloud". They also can follow any legal retention policies you may need.