If mailbox.org gives you new or additional MX records, you can enter them in the domain assistant. To do so, you switch the mail server to a custom mail server, because mailbox.org is treated as a custom mail server there.

Prerequisite

The affected domain must point to a client. Only then are the menus for MX records enabled.

How to enter the MX records

1. Open the domain assistant under Setup » Settings » Hosting » Domains.
2. Under Selection of advanced settings, activate the mail server.
3. Select a custom mail server instead of Mailbox.org.
4. Enter the IP address of the mailbox.org SMTP server that you use to send emails.
5. Enter the MX records that mailbox.org gave you.
6. Save and complete the assistant so that the update is carried out.
7. If necessary, let the domain point to external again afterwards.

Further information in the manual: https://knowledge.plentyone.com/en-gb/manual/main/business-decisions/domains.html