What is SMTP UTF8 and does it affect authentication?
SMTP UTF8 defined in RFC 6531 allows internationalized email addresses also known as EAI or Email Address Internationalization that include non ASCII characters. This includes both the local part and the domain.
SMTP UTF8 itself does not change SPF DKIM or DMARC logic, but it does introduce complexity:
Headers may require different canonicalization.
IDNA encoding affects domain alignment.
Some older MTAs reject SMTP UTF8 entirely.
Authentication continues to function normally as long as the MTA implements internationalization standards correctly.
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