A great piece of detective work. I'd never have suspected that a web-based business sending me email ads could get inside the Mac and generate official looking keychain requests. They've stopped now that I've upgraded to Lion (although perhaps that is coincidental). I've also just recently unsubscribed to Smith Micro's emails.
But, if you are correct, I'd have to say this is a major breach in Apple's secruity sceme - perhaps harmless in this case, but certainly a vulnerability that could be exploited by the so inclined.
Just a fiew moments ago I had the same problem.
Mail showed my the pop-up you discribed after trying to send an e-mail.
Without thinking I clicked Agree and afterwards I searched in my Keychain for this mysterious InternetAcount Access Group, there I found under the label of the SMTP-server of my main e-mailadress. It said that InternetAcount Access Group was free in using this data. I've immedialty deleted this label from the list of programs that where allowed to use my data. Afterwards I deleted the label which it had made.
The same e-mailaccount on an other computer was using a different SMTP-server so I changed SMTP-adress in my SMTP-service list into a new adress.
I think Michael is true in his conclussion that it could have bin a e-mail whit some kind of virus. The problem doesn't show up now anymore, now I've changed the SMTP-adress... I hope you haven't got problems with this pop-up anymore and that this is only new advice for other users of this page.
P.S. I'm Dutch and I guess I've made a lot of grammatical mistakes, please forgive me for that!
Just in case someone else stumbles on this thread as a result of looking up InternetAccounts, this is the standard Keychain item that is added when a user configures an Exchange account through System Preferences. Apple Mail, Calendar, Contacts, and Notes all pull from the same InternetAccounts keychain item in order to authenticate to the server listed. It is NOT malware/virus but a standard element of the OS.
I have had this problem for awhile with popping up with that message when emailing photos. I went into I-photo preferences and turned off the option of "share entire album." The message no longer pops up. I don't understand what the "sharing entire album" means but I am happy I found the solution for me.