About This HTML Issue
This mailto: link has a tab character inside its href, and a URL can't contain a raw tab.
A tab is hard to spot because editors draw it as blank space, so the value can look like it has an ordinary space after the colon. It usually comes from copy and paste, for example from a spreadsheet or a table, where cells are separated by tabs. The validator says the tab comes "after ilto:", which means anywhere after the scheme: right after the colon, inside the address, or after the comma between two recipients. The character reference 	 is decoded before the check, so it triggers the same message.
A tab at the very start or end of the value isn't reported, because leading and trailing whitespace is stripped from URLs. A tab inside the subject or body parameters gets a different message, "Tab, new line or carriage return found." To put a tab in the message body, encode it as %09.
Browsers remove tabs from URLs before following them, so the link usually still opens the mail client with the right address, but the markup is invalid. Delete the tab so the address follows mailto: directly. If you can't see where it is, search the source for 	 and for tab characters, or retype the address.
Invalid example
<a href="mailto:	info@example.com">Email us</a>
<a href="mailto:info@example.com,	sales@example.com">Email sales</a>
Valid example
<a href="mailto:info@example.com">Email us</a>
<a href="mailto:info@example.com,sales@example.com">Email sales</a>
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