the user addition code needs to at least do a lookup of the host name to see if it exists before it tries to add the user.
added changes to lose any spaces that were accidentally inserted into email addresses (see bug #1223). while this wasn't really a bug, and has code to explicitly deny email addresses with spaces in them, it was a simple thing to add.
Backed out changed for #1223 because it was causing "oddities" that it shouldn't have. I choose the lesser of two evils.
This is just too difficult to do; there's no concrete way to ensure that hostnames exist with DNS servers going up and down all the time.