Hello Mirek,
I updated the QPEncode() function using the original code snippet in SMTP. It turned out that the coversion of HT and SP was implicitly present, I guess I should read more carefully next time.
Anyways, my previous version of QPEncode was working but this version is more compact (unnecessary code removed) and also encodes the last space character right before the EOF to QP, if no CRLF present at the end.

Regards.