How to set up server side message filters on sendmail/Imap server

A quick google throws up IMAPFilter which looks like it does what you want:

• Searching of messages using many available criteria in the IMAP protocol, such as:
  • Status (recent, unread, etc.) of a message.
  • Size of a message.
  • Age of a message.
  • Matching of a string or a regular expression pattern in the headers or the body of a message.
• Ability to use logical operators (and/or/not) while searching messages.
• Different capabilities available to process messages, including:
  • Deleting messages.
  • Moving messages to a mailbox in the same or different servers
  • Copying messages to a mailbox in the same or different servers.
  • Marking messages or changing message flags.
• Listing of the available and/or subscribed mailboxes, along with support for the IMAP CHILDREN extension.
• Creating, deleting, renaming, subscribing or unsubscribing mailboxes
• Internationalisation (I18N) support.
• Server namespace support using the IMAP NAMESPACE extension.
• Secure Socket Layer (SSL) or Transport Layer Security (TLS) encrypted imaps (port 993) connections.
• Encrypted connections using the IMAP STARTTLS extension.
• User authentication with the Challenge-Response Authentication Mechanism (CRAM), specifically CRAM-MD5.
• Perl Compatible Regular Expressions (PCRE) support.

How do I setup IMAP (client and server) so that every client using the same email account has the same folder structure.

If all clients are accessing the same account then their folder structure will always be the same. This is because all folders are stored server side, so there is only ever one master copy.

Saying that, you can affect the "view" of that folder structure by what is called subscriptions. Which is a record of which folders a client wishes to see. However most subscriptions are also stored server-side. So if all clients are connecting to the same account then their subscriptions should all remain the same.

These is an IMAP alternative to this called shared folders. Which allows clients with different accounts to share a folder from another location. But if you're not providing the IMAP server yourself then it may not be available. So we'll go no further with that.

Our server mail setup doesn't support email rules ("if sender is foo, move email to Important"), so there is no way to centrally and automatically send emails to certain folders. Is there a way to solve this? We are using Outlook Express on Win XP PCs, but I rather not create the rules on each client as this is cumbersome to maintain.

Without the use of server-side filters there isn't much you can do about this.

Should we migrate from Outlook Express to another client that isn't tied to Windows (we're currently using Win XP on all PCs)? I mean, Outlook Express has been replaced by other software in newer versions of Windows so using, say, Thunderbird might be safer?

Outlook has never been a great IMAP player. OE's implementation of the IMAP spec has always been OK. Certainly it performed better than straight Outlook for a very long time. Outlook 2007 was the first straight Outlook to really work, having finally added support for server-side Sent Items. But there are still a heap of things missing. Such as the ability to use other server-side builtin folders and define delete actions.

Thunderbird does however perform very well. The biggest blocker for it's uptake (IMHO) is it's unfamiliarity to Outlook users and lack of corporate image. If you're able to make the change though, as in your users won't mind, then absolutely do so.