badbadmonkey - 11:50 am on Aug 11, 2011 (gmt 0)
These are not direct substitutes, "inquiry" means something slightly different in English English. You have a formal inquiry while a customer makes an e-mail enquiry. For this reason you should stick to the latter in international usage, as the Merkins will understand it just fine and you won't cause frowns on the faces of the rest of the world.
I've used enquiry as the main e-mail address for years, i.e. email@example.com, with no problems. I do redirect from inquiries@ and other misspellings but never any actual complaints (!) - I wouldn't think much of anybody who bothered to quibble on something like that.
If you're a British company I would stick with British spelling and go with that identity. Your "voice", style of writing etc, is likely to come across as slightly "foreign" to US visitors anyway. If you do want to manage the variant spellings, you would need to implement geo location and a dictionary of alternative spellings. Unless you have other reasons to do geo variations I wouldn't bother.