Since noone seems to have a solution for this and upgrading to Indy 10 is no option (too many projects are involved) I found a different library to handle smtp with tls...
Now I use Synapse which does not interfere with Indy 9 and does just what I expected.
http://synapse.ararat.cz/doku.php/public:howto:smtpsend
Hopefully this helps others with the same problem.