Marc Egli writes:
Now my question is: can i release my application under a less restrictive license like the bsd license?
Probably not.
According to the gpl-faq this is a borderline case because i only invoke a function and wait for the response. http://www.gnu.org/licenses/gpl-faq.html#NFUseGPLPlugins
The plugin really needs to be designed as a separate program with a single entry point and enumerated options (at most, with simple data types like strings and integers as arguments) to qualify as borderline here. However, the Bouncer API requires two complex data structures (a mailing list object and a message object) to be passed in. That pretty clearly is covered in the second paragraph of that FAQ, not the third.
Sorry. Like many in the Mailman community, I lean to the permissive side of the free software movement. I'd like to say "sure, you can do that." But you'd be at risk of being sued (by the FSF, no less) if you did. IANAL etc etc, and you might want to get a more authoritative opinion from the FSF itself.