65univention-ox.inst failing - can't join domain

I am trying to rejoin the ucs domain on a machine with ox installed. All of the install scripts except for 65univention-ox.inst are succeeding. This one failed (exitcode:1).
There is already a server in the domain that is also running the mail server.
When the new server tries to join it can’t because the other server is showing up owning context 10.
Can I just change the context number or do I need to remove the mail server and reinstall it after the machine is in the domain?

I am seeing the following in the join.log file.

ox-server-install: Checking join status...
ox-server-install: Checking for Open-Xchange context in LDAP

The context ‘context10’ is currently registered as:

DN: cn=context10,cn=open-xchange,dc=xxxxxx,dc=yyyyyy,dc=com
  contextid: 10
  hostname: mail2.xxxxxx.yyyyyy.com
  name: context10
  oxDBServer: mail2.xxxxxx.yyyyyy.com
  oxQuota: 59725
  oxintegrationversion: 9.0.7-11A~

The referenced host is still registered for the service ‘ox’.

I don’t want to delete the existing server which is listed here.

Is OX still installed on mail2.xx.yy.com? If not, you can safely delete the UDM entry (udm oxmail/oxcontext remove --dn cn=context10,cn=open-xchange,dc=xxxxxx,dc=yyyyyy,dc=com).

Or do you wish to install OX on one server and use the other server as its IMAP & SMTP server?
Then you’d have to udm oxmail/oxcontext modify --dn cn=context10,cn=open-xchange,dc=xxxxxx,dc=yyyyyy,dc=com --set oxDBServer=<the ox server>

It won’t take the --set oxDBServer=mail2

This is a lab environment.
We want one sever to service incoming mail and one outgoing mail. The server with incoming mail is working, but when we try to add the outgoing server it fails. Do we not need OX on the server for outgoing SMTP/postfix?
We will have some users on the inbound mail server and other users with mailboxes assigned to the outbound mail server.

What do you want to achieve with this?

No. The mail server is complete with Postfix to handle SMTP (with user clients and other mail servers) and Dovecot to handle IMAP/POP3 (with user clients).

OX is a web frontend to read and write email. It is not a mail server. It communicates with the mail server through SMTP to send and IMAP to read email.

OX is the web equivalent to Microsoft Outlook / Mozilla Thunderbird / Apple Mail / whatever-mobile-app.

What do you want to achieve with this?