non-English characters display issue in gmail, yahoo via phpMailer class

I use phpMailer (as of today version in GitHub) to send automatic smtp activation mails from my noreply@host.com.

I tried it in gmail and yahoo. Both interpreted the characters as shown below.

nice      unwanted (realized)
Ç    ->     Ç
ı    ->     Ä
ş    ->     Åž

mailing process order is:

1. "sign page" (has the form, utf-8 encoded), then
2. "assess_from_sign.php" (pure php without any encoding command. actual sending or failure process is here),then
3. "inform page" (informs user for result)

my message in mail body starts with Dear $_POST['useame']

What can I apply to $_POST['useame'] variable in assess_from_sign.php page so even non-English characters seems exactly like in their own language.

note: all requests are redirecting to index.php page in my site and it has mb_inteal_encoding("UTF-8"); command which applies to all pages.

thanks, regards