My mailbox gets full frequently despite constant archiving. I check the folder size using my mailbox properties to help me clear the problem areas since it shows the distribution of the data amongst the server folders. Most folders get cleared out, but one area that starts to build up over time in my Server Data’s folder list is the “Sync Issues\Conflicts”, which I could not clear.
Today I finally found the folder in “Go > Folder List > (left navigation panel)”, and managed to clear it out. To go back to the original folders, click “Go > Mail”.