Just a quick post to save other Partner having the same experience I've just had.
Apparently OpenSRS now has limits set on sending emails for new email addresses. It doesn't matter whether it's an existing domain/website, as soon as they set up a new email address in the BC system, they are limited to a miserly 10 emails/day. This will slowly work it's way up to 25 - 50 etc (see screenshot from support of one of my client accounts - http://screencast.com/t/Pp1oKRkS5I).
I've had 2 different clients contact me in the last week about not being able to send emails and after days on support tickets and an hour on chat, I have found out that there is indeed a limit. It seems that if you contact support and show your displeasure, they can contact OpenSRS and get the mimit increased (just not immediately).
I looked like an idiot telling my clients that there must be an issue with their Outlook setup, when all along the issue was with BC.
Maybe I should change them all over to Google Apps.