Why can I no longer send emails via the built-in email form

It comes to my provider and then nothing more happens

I have one.com as a provider

it has been working until quite recently and I have not actually checked it.

Is it an update from you or is it one.com that has a problem

They claim that it is because I have a return path that is not equal to my sender all return path from them is postmaster and then I can not just see the idea in being able to forward mails

Jann