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Email Authentication Fundamentals
DKIM: Who Signed It?
What characters are permitted in a DKIM selector?
What characters are permitted in a DKIM selector?
Updated over a week ago

The DKIM standard requires that DKIM selectors only include letters, numbers and hyphens where a hyphen can not be the first character. Dots can act as separators between labels. A few vendors have begun using underscores in the DKIM selectors they generate for their customers. While these are non compliant with the standards, many email receivers will process these correctly. Depending on how strictly the receiver follows the DKIM standard, underscores may work in some, but not all, cases.

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