Had not expected 'underscore' to be used in an email address, the common_types.xsd has the following string pattern defined as "" which allows Uppercase and lowercase alpha characters, numbers, @, fullstops and hyphens as valid characters only.
Looking at the requirements in the ILR Spec, the only validation is about having an @ and fullstop separators.
Googling Wikipedia – http://en.wikipedia.org/wiki/Email_address#Invalid_email_addresses
There is a lot more valid characters that could be used in an email address, though as noted Hotmail has it's own restrictions on what it will allow.
If the group finds more Valid Email Addresses formats that the XSD fails – will need to consider relaxing the pattern validation in the XSD.