well... you are coding something created by YOU.
there's nothing illegal in coding any kind of emulator since it's nothing stolen.
i've got some friends who got similar letters from game companies, but they failed in suing people for developing emulators.
feel free to go on with the coding

i suggest you to google some stuff about copyrights etc
your created emu may have the same features as the original gameserver, but that doesn't matter since it's not copyrighted by them.
one example:
take some kind of file converter for example.
there are about 50 file converters arround (some free- and some shareware), they all do the same things ->converting files. they won't be able to sue each other since they are using self-coded code. The effect of the compiled work isn't under any license.
anyway.. this doesn't make sense at all..
look at eternigames,egq,extreme. they all are using the stolen dekaron files (eternia injected some dlls in order to fix the new buffs etc).
They don'T own any rights either. and they didn't get any letter.
this may just be a prank