Symfony version(s) affected

5.1

Description

Commit d057dff allows non-ASCII local-part in email addresses but this should come with RFC6531 implementation (SMTPUTF8 extension).
It seems that this is not implemented.

How to reproduce

$transport = new EsmtpTransport($host, $port);
$mailer = new Mailer($transport);

$email = (new Email())
    ->from($from)
    ->to($to)
    ->subject($subject)
    ->text($recipient);
$mailer->send($email);

If you try sending an email with non-ASCII email address to a server that correctly implements RFC6531, it will return 501 Syntax Error.
You can try with this version of Mailhog that implements SMTPUTF8 extension.

Possible Solution

Implement RFC6531

Additional Context

No response