iOS error sending email

Hello!

I'm new to mailbox.org.

I've enabled my profile last week and I've managed to set up all my devices (Thunderbird on macOS + Apple Mail on iOS)

I've also enabled a custom domain with multiple aliases.

Unfortunately, I've had a problem sending emails from iOS using Apple Mail since this morning—no problems with Thunderbird.

The issue affects the main email address and the aliases created with the custom domain.

Attached are the images about the error and the settings of the email account (IMAP)

Any thoughts?