Did’nt solve the issue aswell.
But i realized that the whole website did’nt send mails (forgot password etc.).
So it seems that it is’nt a cf7 related problem but a wordpress 5.7 issue.
I changed the sending method to smtp and it’s working now. I’m still having no clue what the problem was though. Especially as i have running very similar projects (same plugins, versions, theme) without problems.
However, thanks for you effort.