Thanks for responding and good to hear from you again, Imar!
To be sure, I just opened the site and sent myself the email containing the link. It looked like this when it arrived in my email box
http://abc123:4545/surveys/frmSurvey.aspx
The link opened the page successfully from my email client. So it's composed correctly by the .Net code and it is not being changed by SMTP on the IIS server when it is sent. Consider also that out of a user base of several thousand, this is the only user to report this problem (so far). It therefore seems likely to me that some Outlook setting on her workstation is modifying the link, even though the user's settings are supposed to be identical to mine. And just to make this more interesting, this user is an executive working a thousand miles away!
Anyway, I already posed the same questions to the user and am awaiting a response. So I'm no more enlightened than you at the moment, unfortunately.