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The domain name part of an email address has to conform to strict guidelines: it must match the requirements for a hostname, a list of dot-separated DNS labels, each label being limited to a length of 63 characters and consisting of uppercase and lowercase Latin letters A to Z and a to z; digits 0 to 9, provided that top-level domain names are not all-numeric; hyphen -, provided that it is not the first or last character. [from Wikipedia]
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