As already discussed and not resolved here: http://community.office365.com/en-us/forums/172/t/57936.aspx

I would like to see a warning when a user attaches a large attachment. There is currently none, and the user thinks their email has been sent, when in fact it tries to send over and over again. I've seen users inadvertently attach files that are 100's of MBs in size. I've also seen it use a lot of data as the system tries to send these emails over and over again.

I believe there is such a feature in the Windows version of Outlook – is there a plan to add it into the Mac?