>>>Also, You can bring up Outlook, and from it's Tools\Options menu, the Other tab may be what you need.
>>
>>Didn't see the last phrase before, you probably added it later. I'm using Outlook 2000 and it doesn't have that option :( Well, it's my punishment for not being up to date :)
>
>Yep, I snuck that one in later. I don't remember what Outlook 2K looked like, we're using Outlook XP (2002) here at the office. I just use OE at home.

I should not be that lazy! This page http://www.opera.com/support/search/supsearch.dml?index=433&session= seems to address my problem:

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If you wish to use Outlook 97/98/2000/XP as your e-mail client, you need to enter a special command line option when configuring this in Opera.

Open File > Preferences > E-mail and choose "Use specific e-mail client". Then click "Choose" and browse to locate the executable file for Outlook (Outlook.exe). This file's location is usually C:\Program Files\Microsoft Office\Outlook.exe.

When the file path has been entered into the field, you need to add a command line option:

/c ipm.note /m "%t&cc=%c &subject=%s&body=%m"
The path should now look something like this:

C:\Program Files\Microsoft Office\Outlook.exe /c
ipm.note /m "%t&cc=%c &subject=%s&body=%m"
Please note that the path to the executable file can be different on your computer.

These settings will insert the e-mail address into the "To" field in your "Compose message" function in Outlook 97/98/2000/XP when you click a "mailto" link in Opera.