How do I send mail from a configured custom domain e-mail address?

I configured my custom domain 'example.com' and added mail addresses like 'johnsmith@example.com'.

If I send a mail from an external provider (eg. gmail) to 'johnsmith@example.com' it arrives just fine and I can reply to it from the mail address 'johnsmith@example.com'.

However if I compose a new mail in mailbox.org I can only select @mailbox.org ending e-mails as a sender and not my custom domain, eg. I can send from 'johnsmith@mailbox.org' but not from 'johnsmith@example.com'.

What do I miss, is there a setting for that somewhere?