Hi Daniele
I have checked this again with MailPoet V3. The latest X/Pro theme compatible MailPoet 3 premium version is v3.0.45 as you can see here on our version information guide (https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195).
Then, since you’re using V3 now, you need to select the correct version and create your API key.
When validating the sending service key, I still see the first error.
But you should be able to activate your MailPoet sending service under Sending With tab.
If you want to start over, you can head over to the Advanced tab and then click on the “Reinstall now” button. Please note that, this step will erase all of your existing data.
Let us know how it goes.
Thanks!