Permissions
In Microsoft 365, permissions can enable an individual to access another individual's mail, calendars, and contacts. For example, if User A wanted the ability to modify events in User B's calendar, User B would need to assign read/write/modify permissions to User A. Review complete list of permissions levels (and definitions): Microsoft 365 - Mail and Calendar folder permission levels.
POP3/POP
Post Office Protocol (POP) is an application-layer Internet standard protocol used by local email clients to retrieve e-mail from a remote server over a TCP/IP connection.
Primary Address
In Microsoft 365, your primary address is the email address associated with your Microsoft 365 account that will appear as your "From" address on emails and calendar invites. It will also appear as a result in the global address look up feature in Microsoft 365. For further information, click here.