IIS FTP authenticates with domain\username but not username@domain.local

I have an FTP setup on IIS and I can access it by typing in domain\username, but when I try to authenticate using username@domain.local it fails.  Any idea why?
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Steve BinkCommented:
What happens if you user username@domain?
What version of IIS are you using?
jdouthitAuthor Commented:
I'm using IIS 7.  If I use username@domain it will not authenticate.
Jian An LimSolutions ArchitectCommented:
after speaking to a few of them, this is by design.

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