I travel quite a bit with my laptop and have no facility to send 'Out
of Office' replies to people who send me messages.  When I travel, my
laptop is offline, I do not have an Exchange-based server to do this,
and my ISP does not have an easy-to-use option (that I can discern) to
help with this.

I am looking for a tool that I can use that will allow my e-mail
messages to be auto-responded with Out of Office messages.  Can anyone
help?

You need to sign up to a mail service providor that has a web based
out of office notification service.  If you have your own domain you
need to move it to their servers.  Usual price here is about 20.00 GBP
for a .co.uk address

Our office use proindigo.com.

I think you could do this easily by using the Rules Wizard.

You could create an email rule which would apply for ALL incoming
messages, which would check the recipient address for your email
address (so that the rule would apply to messages which are sent
specifically to your address).

Then where the rule lets you choose "What do you want to do with the
message?", check the "reply using a specific template".  You'll need
to create a template message (for example, "Sorry but I'm out of the
office until 7/7/2005, if this is an emergency please call 911.").

Once the rule is created, you can activate it by clicking Tools =>
Rules Wizard, and then checking the rule definition in the list; a
checkmark beside a rule means it is active and will fire when a
message meets its criteria.

When it's active, the rule will cause Outlook to automatically reply
to the user and send the template you define.

The only two drawbacks I can think about right off when compared to
the Out of Office feature with Exchange is that a) Every email will be
replied to, unlike Exchange which will send the out of office only
once to any one sender; and b) Outlook will most likely need to be
open the whole time you're actually out of the office in order for the
rule to activate and fire.

Thanks, zoplax!  Actually, I need to have a service that responds to
the messages while I am offline. But your response has made me make
some sense of the murky instructions in Outlook!