problem with sending mails using outlook 2007

After 3 weeks of trying out a lot of stuff i still can't get my mails to be
accepted by the mailer daemon.  So i ended up using our webmail variant for
which in some cases my mails arrive...

Anyone who can point out instructions to get outlook 2007 configured
properly?

Steps i already tried:

Tools -> options -> mail format
message format has been set to 'plain text'

then when clicking the international options button
-deselected 'auto select encoding for outgoing messages'
-preffered encoding for outgoing messages has been set to UTF-8

Robby Pelssers