Okay, so I changed the sender mail to wordpress@domain…
Now test mails are sent from that address, but what’s more interesting, now the sender name is wordpress. I sent a testmail to your test mailbox as well.
There is a mailbox for nyhedsbrev@domain… but not for wordpress@domain…
I already have another plugin that sends mails, those are always from wordpress@domain… (not configurable).
I have created a WP user for you. You should have received a default WP welcome mail.