I don't recall where I have seen this recommendation. Only recently I had a chance to test this solution.
To fix iPhoto sending email someone recommended running Disk Utility permission verify and repair.
Search for "Disk Utility" and click on it. Select "Macinto HD". You are likely to see Macintosh HD and then the same "Macintosh HD" under it. You care for the nested item. Click on nested Macintosh HD. Click on Verify Permissions. If utility detects there is an issue with permissions, then click on Repair Disk Permissions.
If you have Mac with harddrive instead of flash based (retina display laptop), then it is worth it verifying that your disk is in top condition with "Verify Disk" button. You will be prompted to fix issues if there are any. If utility finds issues it is strongly recommended that you use repair procedure. However, repair procedure can take some time, so you better do it when you have time to wait.
My Finder misbihaved a few days ago and I decided to very disk. There were issues with it, so I had to reboot into Cmd-R mode and then use repair option. After that I used verify permissions and repair permissions buttons as there were issues there as well.
Today I tested my iPhoto ability to email via @me emal address and it worked. It did not work 2 or 3 weeks ago.
I was not trying to address iPhoto issue, but the correlation is there and is worth mentioning.