Testing/checking distribution list ?

Okay, so I just had 94 emails bounce back with illegal host/domain name errors.

It seems that everyone, but two people in the office that I copied, bounced back.
I have used this list before and two or three came back with an error but I don’t see why they would ALL bounce back now. Would ONE address throw the entire list out?

Could it be an issue with the ISP, because it was delivered IN the office but not outside of the office.