User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:93.0) Gecko/20100101 Firefox/93.0

Steps to reproduce:

Try to send email to an UTF-8 address. E.g. "árvíztűrőtükrfúrógép@gmail.com" or similar. It is very interesting, that an address like this is

• valid ( see https://en.wikipedia.org/wiki/International_email#Email_addresses )
• it can be added to the address book
• but it cannot be used as a recipient

Actual results:

Thunderbird does not try to send out the email. (SMTP server is not connected). It just says that there are non-ASCII characters in the local part of the address, and it is not supported.

Expected results:

Send out the email.

Using Thunderbird 102.B8 under Windows 11Pro I tried sending a email from my Gmail account to árvíztűrőtükrfúrógép@gmail.com . I got a Delivery Status Notification (Failure) message in my inbox that reported a 550 5.1.1 error, also mentioning:

Address not found
Your message wasn't delivered to árvíztűrőtükrfúrógép@gmail.com because the address couldn't be found, or is unable to receive mail.

I did not get a copy of the message in my Sent Items folder. I then tried sending the same message (using the same recipient) from my Fastmail account. I got a popup that said:

Sending of the message failed.
There are non-ASCII characters in the local part of the recipient address árvíztűrőtükrfúrógép@gmail.com and your server does not support SMTPUTF8. Please change this address and try again.

In both cases this seems like correct behavior. I have not seen anything official about Fastmail not supporting SMTPUTF8 but I've read threads such as https://support.google.com/mail/thread/87961215/bug-in-implementation-of-idn-for-non-smptutf8-receivers?hl=en which claim that.

This was fixed by the bug 1563891 in TB91