I can receive emails, but cannot send them from one of my gmx email accounts

For a couple of weeks now, I have been able to receive emails, but not send them from one of my gmx email accounts. I have 2 gmx email accounts and despite the settings being correct for both, only one will send emails. I have contacted gmx.com and they confirm that the settings are correct, but the error message is very strange.

This happens on both my iPhone and iPad, but is ok on my iMac. I have tried deleting account and re-adding