Unfortunately, emoticons are covered by Unicode surrogate pair characters, which are not supported correctly in the current version.
This will be fixed in the next release. If you have active support and you want to receive hotfix version right now, please let us know.
For list of available Unicode emoticons see and its representations in different systems visit https://apps.timwhitlock.info/emoji/tables/unicode.
To specify an emoticon just copy&paste from the list above:
var mail = new MailMessage();
mail.Subject = "GRINNING FACE _COPY_AND_PASTE_HERE_";
or use the UTF-8 notation like this:
mail.Subject = "GRINNING FACE " + Encoding.UTF8.GetString(new byte[] { 0xF0, 0x9F, 0x98, 0x80 });