[philote]

At my current employer we do allow this. It's handy for testing and not a big deal if we have users doing this to make multiple accounts (they don't gain anything by doing that). But at a previous job it was a bigger deal and we'd strip off the sub-address part because we were trying to match up email addresses across client sites (this was due to a bit of a shady thing the company was doing and part of the reason why I left).