Greek charset on html mail for yahoo mail?

[Lazaros T]

Well the case is this, I ‘m building a newsletter system and there is a problem with the encoding on yahoo mail, while the html mail displays perfectly in Gmail and hotmail. I have tried to set the Content-Type on the header of the mail, either to utf-8 or iso-8859-7(Greek charset) and the charset on the html with a meta tag. Could you please suggest something else that I could try?