iOS Mail is stuck in Outbox for iCloud

I can receive emails send to my @mac.com address.

as soon as I send one with this @mac.com it is trying and then stuck in outbox.

the settings look fine, they are done automatically logging in with my apple Id.

I am stuck😥

I already delete the iCloud account from the device and added it fresh.

i restarted the device.

an additional account works just fine.

using safari and the same account to log on to iCloud works. I even can send an email from the browser.

using latest build 10.3.3.

Michael