acn128 There are other such software packages (not similar to grommunio, other categories) that have a similar setup. Where they have an absolute free version, but then have other tiers for home users who need other services, but don't need the full enterprise version. Such software companies offer multiple tiers for the community versions. They include a monthly, yearly, or lifetime license.
But in your specific use-case, these "functional mailboxes" -- you can create mailing lists.
In my testing, what I did was create a generic "service" account as a regular user (which did take up one of my 5 user slots. Then I create mailing lists for each service. For instance, I run Gitlab, Jenkins, and Ombi. I created a mailing list for each gitlab, jenkins, and ombi. Then inside the software, I configure the mail-from address, for instance in jenkins as 'jenkins@domain.com'. But since Jenkins needs to be able to send emails, it must log into the mailserver to send the email, I set the login credentials to that of the 'service@domain.com' and the password. Now jenkins can authenticate to the mailserver, and send the email as 'jenkins@domain.com'. This has worked for me so far.
But I have run into applications that you cannot set a mail-from address, it uses the account you have setup, and in those cases, the mail comes from 'service@domain.com'. This is less than idea, but still acceptable since these programs do not receive emails themselves.
But in the cases where the services do accept emails, and can parse the emails for specific commands and such, this does not work. I do have a few applications that do accept emails and would log into the mailserver to check mail and process said emails and react based on the email. In this situation, mail-lists would not work, and a catch-all service account does not work.
This is why I purposed a Community Plus edition. Maybe have it tiered.
Community - 1-5 Users - Free
Community Plus - up to 15 Users: $15
Community Plus - up to 25 Users: $30
Community Plus - up to 50 Users: $50
50 users sounds like a lot, but when you factor in said service accounts for those of us who use other software like gitlab, jenkins, ombi, [own|next]cloud, plex, emby, jellyfin, docker, grafana, and any other the other selfhosting software out there, this isn't a high amount.