That's one of the most generic error messages you can get, and it's very frustrating. That is one of MANY reasons NOT to use Outlook, but, well, if you're committed to it, that's that.
Have you first made sure that your connection is O.K.? Is your browser working properly? Have you made any changes AT ALL to the Outlook mail settings?
Have you made any changes--EVEN THE LEAST--to any of the groups, or to any of the individuals that are in the groups? The least error in ONE recipient's address, like a missing dot, can invalidate the whole group. Then, you have to go piecemeal to find out where it is.
My mother once had a group with 100+ people, and "something was wrong." We had to divide it into two groups of fifty, and we found that the second group was the culprit. We then divided THAT group into two groups of 24, and found that the FIRST subgroup was at fault. Etc., etc., until we had few enough addresses that we could eyeball them and say, "Oh, looky that, we forgot the '@' symbol."
I don't mean to give you oodles of work to do, but have you tried this?