Symfony version(s) affected: 5.3.1 and older

Description
We recently moved from Swiftmailer to the Symfony mailer and we have issues with sending emails to domains that contain non-ASCII characters.

Example: something@öäü.com

Our local postfix receives this email address without any Punycode encoding end gives an error like

> RCPT TO:<something@öäü.com>\n
< 501 5.1.3 Bad recipient address syntax\r\n

I had a look and it seems the Symfony\Component\Mime\Encoder\IdnAddressEncoder is not used in this case to encode the addresses.

The SmtpTransport is just using the raw address: https://github.com/symfony/symfony/blob/5.3/src/Symfony/Component/Mailer/Transport/Smtp/SmtpTransport.php#L199

However the SwiftMailer transport was encoding the address first: https://github.com/swiftmailer/swiftmailer/blob/master/lib/classes/Swift/Transport/AbstractSmtpTransport.php#L383

With Swiftmailer the command looks like:

RCPT TO:<something@xn--4ca9at.com>\n

So I'm wondering: Is there a reason we are not encoding addresses anymore?