I suspect that this issue may not be a Nintex issue. but I'll try here anyway. I have a list that has a People / Group picker field. Works great when I select a person or a group on SharePoint. However in our company we have email boxes that are shared. For example one of the shared email boxes (team boxes) is firstname.lastname@example.org with an internal name of "Confirmation Team".
When I just the people picker to and type in "Confirmation Team" it says it can't find it. When I try email@example.com (that would be considered in domain address)it says I can't share externally. How do I get the people picker to bring up the shared email box? is there a setting on SharePoint? Keep in mind that I have very limited access to SharePoint's inner workings. The company has locked down most of the functions.
Using SharePoint 2013.
As far as I can say people picker is only capable of resolving users and groups. Mailboxes are not treated as users as they cannot log in and stuff. What is the use case of your request?
If it's about the lookup you could use a lookup list or managed metadata column as a workaround to let people search for available shared mailboxes. If it's more about using it in workflows you could just use a text column or again a lookup or just use the mailbox in your workflow actions as CC or so.
Hope this helps a bit.
This may be because on your AD connection for User Profile Service your filtering out disabled accounts. Shared Mailbox accounts are disabled in AD therefore won't get imported into UPS and won't be available to be added to the site via People Picker if this setting were enabled.