Hi @vox
It is Canonical advertising their Ubuntu Pro subscription in apt and Mail-in-a-Box interpreting it as available updates. For a more detailed explanation see my post here: Version 60 for Ubuntu 22.04 is released - #15 by miabuser
There is also an issue on Github about this: System Status Checks misinterpret advertisement as out-of-date packages · Issue #2169 · mail-in-a-box/mailinabox · GitHub