You just sent an email and a few moments later you receive a return email: Undelivered Mail Returned to Sender

The error message stated is:

Unauthenticated email from ecomail.fr is not accepted due to domain's 550-5.7.26 DMARC policy

This message informs you that you have sent an email with an Ecomail email address as the sender but using a non-Ecomail sending server. This practice is not allowed to ensure the security of sent emails and to avoid phishing attempts and scams.

Check your email software carefully, it is very likely that the selected SMTP server (for sending) is not the one from Ecomail.