We’re currently investigating an unusual software defect that causes some email addresses to be identified as invalid. Only a few customers have observed this and of those, only a few contacts in their application.
This software defect requires additional investigation and we do not have an ETA yet. We have elevated this in priority so we will be resolved soon. We do not know of a workaround, either.
No, we’re not intentionally trying to break RFC standards. 
Thanks for your patience and understanding — updates will be posted below.
Update 3/25/2010: *RESOLVED* This issue has been resolved last night during our maintenance release. Thank you for your patience.
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Hi
Thanks for doing this, but it's still not quite right. The behaviour has returned to how it was when I first reported the original problem (so not as bad as it was). I don't know if I can post images here so I'll update my blog post on lizjamieson.co.uk