About This HTML Issue
The href value of a mailto: link contains a double quote ("), which the URL can only hold percent-encoded as %22.
The ilto: in the message is the tail of mailto:, and the check covers the whole address after it, so a " at the end of the address gets the same error as one at the start.
Usually the quotes were meant for the attribute and landed after the scheme, as in <a href=mailto:"info@example.com">. No quote follows the =, so HTML reads mailto:"info@example.com" as an unquoted value and copies both " characters into the URL. On that tag the validator also reports “"” in an unquoted attribute value. If a space follows the stray quote, the value ends at the space and the message shows only mailto:". Without a closing quote, a single " stays in front of the address.
Quoting the address on purpose doesn't help. In href='mailto:"info@example.com"' and href="mailto:"info@example.com"", the URL still starts with a literal " when the validator checks it. A display name in email header style, such as mailto:"Jane Doe" <info@example.com>, fails for the same reason: the URL holds only the address, and the name goes in the link text. For the rare address with a quoted local part, RFC 6068 asks for the quotes as %22, as in mailto:%22info%22@example.com.
Invalid example
<a href=mailto:"info@example.com">Email us</a>
Valid example
<a href="mailto:info@example.com">info@example.com</a>
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