Can’t send mail to the same domain

I have several sites, and CF7 is down on all of them.

Each site is supposed to send the form to an email with the same domain name as the site itself, but that doesn’t work.

As a workaround, I noticed that CF7 start to function again, if I change the email address to one that have a different domain name.

That is… This gives error message:
CF7 at https://www.siteA.com trying to send to [email protected]
CF7 at https://www.siteB.com trying to send to [email protected]

But this works:
CF7 at https://www.siteA.com trying to send to [email protected]
CF7 at https://www.siteB.com trying to send to [email protected]