- manage 403 returned by dimail API when mail domain secret is not valid
- improve some tests
- improve MailboxFactory to mock success for dimail API POST call
- override 403.html to return a nice failing error in django admin
- an error message is displayed on mailbox creation form of frontend
We want people to create new mailboxes in La Régie.
This commit adds integration with intermediary dimail-api,
which will in turn send our email creation request to Open-Xchange.
