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The RFC dealing with mailto links (RFC 2368) clearly states (at the end of section 2) that 8-bit characters in mailto URLs are forbidden. The validator however does validate pages containing 8-bit characters in its mailto links.
The Validator, as the documentation points out, only checks for rules spelled out in the HTML/XHTML DTDs, and those cannot express such rules. This is a known issue, but given that the Validator does not advertize such capabilities this isn't really a bug.