Apple has issued enough KnowledgeBase articles on issues surrounding emailing pictures from within iPhoto that I thought I might summarize two of them here.
1. If iPhoto balks at sending photos via AOL, quit AOL and rename it "America Online" -- the name iPhoto expects to see.
2. iPhoto 4.0.1 can have problems sending pictures via Microsoft Entourage. Apple suggests that updating to iPhoto 4.0.3 or later fixes the problem.