I will say most ticket system work with one email per user/client. (e.g. OTRS, OSTicket, Zammad, Zendesk)
So it seems to be established workflow for Ticket systems. I personally do not understand why, but I think it will be some type of database technology issue on why this method/restriction used.
Possible Workaround
Most of our clients we control their emails, so you can set up alias under a shared email.
Accounts@ > Fred@ > Paul@
Incoming emails will always come in as Accounts user. (you can change user on ticket. All replies will go to same email but lets you split it up a bit)
Also, helpful when someone leaves an organization. So for general emails, I create general users and then use the alias method for staff members, If they use this shared mailbox method. e.g. sales@ Info@ account@
This is not perfect but this has helped me with some clients.
Sometimes you have a CO/Director that works with two companies (two emails) to get around this issue I just add (TLA “Three Letter Abbreviation”) onto last name.
Paul Lastname (TLA)
I do this on last name because it makes it easier when searching within Zammad. Otherwise, you just see two Paul Lastname when searching.
No email users are pretty rare. Most of the time you can request these users to get an email even if it was just a personal address.
I do understand this might not be what you’re looking for but thought it might help. 