In the unit "OverbyteIcsSmtpProt.pas" you can find this line:
StrCat(PAnsiChar(Line), PAnsiChar(SmtpDefEncArray[FMailMsgText.TransferEncoding]));
But it must be:
StrCat(PAnsiChar(Line), PAnsiChar(AnsiString(SmtpDefEncArray[FMailMsgText.TransferEncoding])));
Edit: Text translated, first I wrote in German 😉