I wanted to ask, if theres any possibility to let people register an eMail account with any of my domains? Like from a webinterface or by any other means?
No, there is no such feature. If you believe that this is something, many users will want to have, you can try to file a feature request here: Feature Suggestions: Top (1363 ideas) – Your Ideas for Plesk
However, in most cases such a function should probably individually controlled by a software. For example you could create your own PHP script that takes form data from a visitor and uses the visitor's data to run a Plesk shell (client) program that creates the desired mailbox.