It is especially important that the queuing between the frontend and backend be monitored for blockages and other problems, just as the server monitors its internal queues. This is achieved by mapping native source recipient addresses into Internet domain form, then mapping back out to the destination native form. Since all routing is done per the directory, this will be invisible to the administrator. All proprietary systems maintain their own address books and user directories. SMCS is responsible for gathering these directories, merging them, resolving inconsistencies, and then exporting the master directory to the proprietary systems.
Mail use different character sets for text, and different encoding formats for bitmaps.
Other systems have limits on line length, or the maximum number of characters in a text body part.