Reset the default by going to Control Panel-> Internet Options-> tab
Programs
Or try
Start-> Run; outlook.exe /checkclient

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Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
http://www.howto-outlook.com/
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Real World Questions, Real World Answers
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> I've set outlook as my default by going tools > actions > other and
> ticking
> the box. However I still get messages popping up saying I havn't set it as
> default even though the box is ticked. I have four IMAP accounts set up on
> Outlook. How do I stop this box from popping up? I am able to send/receive
> email from my IMAP accounts on Outlook.