[nawitus]

After a quick glance at the standard, I belive email providers like Google who support SMTPUTF8 could support having two email addresses transparently to the user.

According to the standard: "If the message cannot be forwarded because the next-hop system cannot accept the extension, it MUST be rejected or a non-delivery message MUST be generated and sent."

Google could preserve a backup ASCII email for all UTF-8 email addresses, which would be used in the case the recipient doesn't support SMTPUTF8.

Of course, I don't know if Google has plans to support this.