smtp problems with iPhone mail on ios18

Hardfacts:
Iphone 15 pro
iOS 18.3

I have several email accounts. everything was working perfectly until I was forced (Reset Iphone) to upgrade to iOS18
all accounts (google and IOS) work fine except my business domain.
I receive via IMAP but when I send emails, I get a notification, that the receivers address is incorrect.
I walked through second level at apple support.

the same smtp on MacBook works without any problems.