What exactly have you connected to Strato, is it email domain of hosted wordpress? What domain is the email on? is The DNS for the domain setup for email with an MX record and DKIM, SPF etc. Has it had time to propagate? new domains are far from instantaneous.

Most of the folk I have seen here over the years making noises like you are have signed up for something other that a full hosting deal, or have not configured DNS for the domains email.

I am not even going to try and answer you questions because I am over guessing without relevant information, but you might want to disable the experimental new account hub in settings and see if that has an impact. Search experimental in settings to find it.

I use a WebPack Basic from Strato and use it to send regular emails to my transferred domains. Strato's webmail works perfectly.

I've now manually edited an old email in Thunderbird (formerly Host Europe). As of now: It seems to be receiving emails. I still need to check the sending configuration. (change only imap, username and pw)

But that can't be the solution. I actually wanted to keep the old accounts as a backup. It's been about 16 hours since I set them up.

As a test, I set up a new gmx account, which seems to be working without any problems.

ok, experimentell of and it looks good