HTML with strange characters

Any on knows why, when I generate a HTML email the result appear with these little strange “black question marks” ? They are not in the source HTML code.

Look the image:

This should answer your question:

Great! Could you suggest me how can I convert the encoding to UTF-8?

Like the article says:
Assign an encoding to your string with or pass a TextEncoding if you are using a feature like BinaryStream that supports the encoding. Using Text instead of string may also be helpful, although text is a bit slower. But it always contains a defined encoding.

Also a good read :