Our current limit for attachments sent to distribution groups is 512KB + about 30% to allow for encrytion. Some of our distribution groups have 700-800 people and one has 11,000. We are in the process of reviewing our attachment size limitations and looking at increasing the size. What are realistic attachment limitations for large Distribution Groups? How effective is Single Instance Storage? What parameters should I be looking at for sizing? Currently we have a single Hub Transport/CAS server, so it handles quite a bit of traffic, although it's not over stressed. The server is a dual quad-core with 8GB of RAM.Thanks,Dave

Dave, I'm surprised nobody has replied to this yet. I think your question may be so precise people are afraid to be wrong! As we discussed offline, you should be fine with increasing this limit. 512k is such a small number.I dont think there is anything you can really call a rule of thumb on this topic. I would use perfmon to see what happens when you gradually increase the number. You can use Loadgen to test this.also SIS is obviously broken when the recipient is on separate databases, so that would depend on how users are distributed. but I wouldn't count on SIS. its de-emphasized in Exchange 2007 and will likely not be in future versions.for everyone else's benefit:More on Single Instance Store for e2k7http://msexchangeteam.com/archive/2008/02/08/448095.aspxMike Crowley: MCT, MCSE, MCTS, MCITP: Enterprise Administrator / Messaging Administrator