Outgoing mails on iphoneX don't be sent

GW18.1 newest Version is installed on a oes2018-server, GMS18.1 is installed on a sles12sp3 server
All works very good, only the mail keyin on the iphone will not be sent over gms
I don't find a possibility to allow this in gms admin or groupwise admin
What must we do to allow sent messages to internaly or externaly users?

Thank you for your help