Since 2017, the validation has included checking e-mail address validity using the email-validator NPM module. Unfortunately, it doesn't properly handle IDNs, and rejects any e-mail that does not limit itself to US-ASCII. As a result, the validator rejects some correct Signed-Off-By lines.

Even though the problem has been reported upstream more than five years ago now, there's been no response.

Note that this is different from #183, which is about lack of Unicode normalization.