Hello,
I do not know whether you already looked up the RFCs concerning SMTP. You can find links to the RFCs at a wikipedia page:
http://de.wikipedia.org/wiki/SMTP
The RFCs 2821 and 2554 should be interesting for you:
http://tools.ietf.org/html/rfc2821
http://tools.ietf.org/html/rfc2554
If you search for the MAIL command you will find further information. There are also error codes an their meaning included.
Maybe is one problem that you do not put the email addresses into angle bracktes <>.
Maybe the problem is that you use authetification but do not use the AUTH parameter to the MAIL FROM command (RFC 2554, chapter 5)
Maybe the command is not supported by the server.
Maybe you should try to leave this command blank.
I hope this information will help you.
Regards,
Torben