Hi
Does the problem appear if the user that try to send a mail got only one outlook profile ?
To rule out if it's a Outlook problem or caused by your application I would do a simple test;
Log into the server with a user that got the symptom. From there create a new text file on the desktop and Righ-click on it, and select send to -> Mail recipient.
If Outlook ask you your profile, please update it and try again, it point to an Outlook problem in it's configuration.
If it don't ask you for a profile then check your application how it call Outlook, sometime they can specify a profile to use, but you might have to ask the support of your application.
Thanks