This article describes how to send authenticated email via the MPmail outbound relay, which you would do if you are unsure if you are sending from a static IP address. This can be used for a service or a device.

In the MPmail control panel, select the customer domain from the scope selector at top right, and click into service dashboard.

In the right pane, at the bottom create a secondary environment of type Hornet.email, using whatever name you find suitable.

Click into customer settings, mailboxes, and add a mailbox in the secondary environment.

The parameters required will be the following:
Outgoing SMTP Server: smtp.hornet.email
Port: 587
Encryption: STARTTLS
Auth: password, normal
User: <email address>