In sites that use the e-mail archive, if you add an account to the participant list and allow messages to be sent, and another account exists that uses the same e-mail address but is not a participant in the site, messages sent by the owner of the first account may fail. It seems like Sakai checks the address to see if it has permission to send messages through that site, and immediately fails on the first "no" that it finds. It should check the permissions of all accounts that have that e-mail address, if one of them is allowed to send messages then the message should go through. There is no clear indication of why the message bounces (e-mail error says "not permitted" but the archive tool says the address can send messages).
It may seems stupid to have more than one account with the same address, but it's not. We have encouraged faculty to create guest accounts for themselves so they can log into their own course sites with student-level access to check that things look and work correctly. It's very easy and convenient to simply give the same e-mail address for both usernames instead of creating a yahoo or hotmail address, and the system does not prevent it. If this is a "wontfix" issue, then a new bug needs to be opened to prevent the same e-mail address from re-used by more than one account.
Two workarounds - either add all accounts with the same address to the sites, or always use a different e-mail address for multiple accounts.