"Disappearing passwords" could be a bug in Arachne or a bug in your ISP,
mysterious to me.
"Relaying denied" could be a failure of SMTP authentication. Normally SMTP
authentication would be used by a free email service such as Telebot that is not
an ISP, to prevent an open relay that could be used by spammers. ISPs normally
can check to see if the SMTP user is connected to the ISP, hence an authorized
user. Maybe onetelnet.nl can't do this or is not aware of the ability? Neither
Arachne nor UKA_PPP as of 1.7x1 has the capability of authenticated SMTP.
I recently downloaded rfc2554.txt, 21155 bytes, dealing with authenticated SMTP,
believe I started at http://www.mailops.com
Just found it:
http://www.mailops.com/Help/SMTP/rfc2554.txt
Maybe one could create a Perl script for authenticated SMTP, though the DOS port
of Perl would not be Internet-ready.