What it is
SMTPUTF8 permits UTF-8 in mailbox local parts and extends SMTP negotiation for internationalized addresses and headers.
Why it matters
ASCII-only validation excludes legitimate addresses, but partial support can accept an address that a later system cannot send to or display.
How to implement
Preserve Unicode, validate syntax without inventing narrower rules, store sufficient length, negotiate SMTPUTF8, and use IDNA consistently for international domain names.
Common mistakes
- Lowercasing or normalizing the local part destructively.
- Applying website-domain validation to a full mailbox.
- Advertising support while a bounce processor is ASCII-only.
Verification
Run international-address fixtures through sign-up, database, API, queue, MTA, webhook, suppression, export, and support tooling.