Can you authenticate your domain at Jimdo?
If your domain is hosted at Jimdo, you cannot authenticate it for Laposta. Although Jimdo offers a few DNS options, you cannot configure every CNAME you need there: you cannot add the record for bounces (eprov → mail.eprov.eu).
Jimdo is the only hosting provider the helpdesk has encountered with this limitation. With all other providers, you can simply add both records. The steps for doing so can be found in the instructions per provider.
Contact the Jimdo helpdesk
The more customers ask for this, the more likely it is that Jimdo will make it possible after all. You can copy and use this email for that purpose:
Alternatives
If you want to keep using the same domain name in Laposta, the only alternative is to have the DNS managed by another provider. This is complicated and technical. You can also register a different domain name and use that for your newsletters. At other providers, the DNS is easy to configure. The helpdesk hears good things about TransIP, Mijndomein and Hostnet, among others.
Once your DNS is hosted with such a provider, you can follow the standard procedure to authenticate your domain step by step.