Just to make sure, this is how you'd actually propagate those settings:
- Log in as the System Admin
- Click the Manage icon. (If you're not already there.)
- Click the Manage menu item so that all domains are displayed in the content pane
- Click on the Actions (...) button
- From the dropdown, select Domain Defaults
- Make any changes you want. For example, if you normally use something like "webmail.domain.ext" you'll want to change the Hostname default to "webmail.%domain%". (no "" of course.) Otherwise, don't change anything. JUST make sure that for Hostname you see whatever you want all domains to use for the Hostname.
- Click the Propagation button
- In the modal, scroll down to the Miscellaneous area
- Check the box next to Hostname: mail.%domain% (or whatever you've changed it to)
- Click the Propagate button
That should propagate the hostname using the naming across all domains in your installation. I just tested this feature twice on 2 separate installations and it worked as expected.