not have Permission to send

Hello everyone,
I want to switch to the EM Client, away from Outlook.

Mail Hosting Kerio at Metanet, Switzerland.
Client version is 10.0.5 8104.

Login is [email protected], but when I change the sender to [email protected] in the Kerio Web Client, I get the following error message.
bmail.xxx.net is a subdomain that is required so that I can use the business mail.

User [email protected] does not have permission to send on behelf of [email protected].

Hosting support cannot help me because the error is with the EM Client, Outlook has no problem sending the mail.

Thanks
Philip