Hey @LoriM welcome to the community 
wow sounds great… well done!
so we can assume its a snap install then? post output of sudo snap list to be sure.
however, from your description your issue is more of an app issue than a Nextcloud issue right?
so the mail module you’re talking about, is that the Nextcloud mail app? if yes, then your “submailbox folder” will be a different mail account or an IMAP inbox subdirectory and will most certainly not be saved anywhere on your Ubuntu server! but don’t worry nothing is lost, its just not visible at the moment and we’ll get to the bottom of it finally 
these are all assumptions because you’ve forgotten to fill in the Support template. Please use this when you request support or else you’re leaving us kinda in the dark 