Quite a lot depends on what names Mitel actually holds the rights to. Since e-smith was originally a separate company it acquired back in 2001, I suspect it still holds the rights to the name "e-smith".
I'm not sure about "SME Server" though. Mitel doesn't seem to be using that name any more, a search on mitel.com indicates it now uses the name "MAS" - Managed Application Server.
If anyone in the development group has Mitel's ear it might be worth checking on their attitude.
Although BMS sounds good, I feel it lacks continuity, and has also been used elsewhere (I'm know I've come across at least two products tagged "BMS").
I quite like SME-Smith, it combines the two prior names nicely (not sure about how best to mix upper/lower case

The most important thing is probably to be snappy and memorable (think Redhat, SuSe, Mandrake - all 2-syllable "words"). That comes back to e-smith or some variant, if it's not a problem with Mitel. I'm not too keen on the hyphen personally, perhaps "ESmith" would work better?
(PR firms get paid a fortune for this kind of rambling

Rick