I found this in a few different places around the 'net - hopefully it's helpful:

Cannot Use MSN EXPLORER or AOL To Send a Remote Assistance Invitation via Email Option

When you use Remote Assistance to invite someone to help you and try to choose the option to use e-mail, you might see the following message when you get to the available options page:

Outlook Express is not set up.


Remote Assistance is either incompatible with the default e-mail program on this computer, or could not detect a default e-mail account.

You will also see an option to set up Outlook Express.



Cause
You have MSN Explorer or America Online (AOL) installed and configured as you default e-mail client. Remote Assistance on Windows XP will not allow to send the activation mail message through the MSN Explorer or AOL messaging system, as they use their own e-mail clients and they are not MAPI-compliant. Remote Assistance Invitation e-mail escalation will only work with MAPI-compliant e-mail programs and e-mail accounts.

Resolution
As a workaround, you could use one of the following options based on how you access your e-mail.

MSN Explorer Use Windows Messenger to invite someone to Start Remote Assistance. In order to do that, follow the instructions on the Help and Support Center, Remote Assistance page, under the Use Windows Messenger field.

-or-

Choose the option to Save invitation as a file, then use MSN Explorer to compose the e-mail to the person you are requesting assistance from, and attach the file to it.

America Online Choose the option to Save invitation as a file, then use AOL to compose the email to the person you are requesting assistance from, and attach the file to it.

MAPI-compliant E-mail application If you have a MAPI-compliant e-mail application installed on your computer, and your Internet Service Provider (ISP) uses POP3, IMAP, HTTP or SMTP mail servers, then configure the software based on the manufacturer's and/or your ISP's instructions.

Note: You can also click on the button Set Up Outlook Express on the Remote Assistance available options page and configure it to send and receive email with your mail servers settings, if available.