User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; de; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 Build Identifier: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; de; rv:1.9.1.5) Gecko/20091121 Thunderbird/3.0 I try send mails to admin@häm.ch which is a punycode domain (xn--hm-via.ch) Sending to admin@xn--hm-via.ch works. I expected this to work, since Firefox does the translation automaticaly (eg, i can enter häm.ch and it gets me to the corect website). Reproducible: Always Steps to Reproduce: 1. send mail to admin@häm.ch Actual Results: sends mail to admin@hm.ch which leads to a "Undelivered Mail Returned to Sender" message saying it can't deliver to admin@hm.ch since the domain hm.ch doesnt exists. Expected Results: sending mail to häm.ch OR telling the user about the invalid Domain BEFORE sending. I use my own mail-server, so i checked the log files. I seen, that the TO address was set to admin@hm.ch (stripping the ä). Im not perfectly sure if its TB or the mailserver stripping the ä (how to test that?), however, it definitively does not try make the punycode domain. This stripping could maybe lead to privacy trouble if the domain without the non-ascii chars is a existing domain not in the hand of the punycode domain owner (eg, in this example, hm.ch exists and does not belong to the owner of häm.ch). The easy workaround for sure is to not use häm.ch but the real DNS name (xn--hm-via.ch) but this is ugly to use and requires 2 different writings for the same domain which could confuse people.

The IDN specification for using non-ascii characters has only recently been developed and hasn't been incorporated into Thunderbird yet (last I knew it was still in draft anyway).