This problem is not exist in English environment, but exists in Unicode environment, especially in Asia like Chinese.
All the wordpress works in UTF-8, including mail, but actually,it has been a problem when sending mail include Chinese words in subtitle, will getting garbage words.
I have already a solution here. Every upgrade wordpress I will modify it again, so I hope this modification can be added in the next wordpress version. It doesn't affect the English environment, but very helpful for Asia Unicode environment.
Line 361 (2.6.2) :
$phpmailer->Subject = $subject;
Change to :
$phpmailer->Subject = "=?UTF-8?B?".base64_encode($subject)."?=";
This is the standard UTF-8 subtitle with Base64 encoding, you may check it out.