if this parameter is not set or is set to null:
When a user shares with an external user, they enter an e-mail like email@example.com, and an email is sent to Stephen at firstname.lastname@example.org. When he attempts to accept the invitation (by clicking the link in the email), he can log in with any account he wants to use. For example, he could use email@example.com, firstname.lastname@example.org, or even email@example.com. The sharing email can be forwarded and accepted by anyone. This system ensures that external users who use email aliases or who do not have a Microsoft account or organization account are able to accept the invitation.
If this parameter is set to true:
The RequireAcceptingAccountMatchInvitedAccount parameter ensures that the user who receives the invitation is also the user who accepts it. If an invitation is sent to firstname.lastname@example.org, only a user who can log into email@example.com is able to accept the invitation. Any other email account displays an error page that directs to user to use the appropriate account.
get-SPOTenant | FL RequireAcceptingAccountMatchInvitedAccount
Ensures that an external user can only accept an external sharing invitation with an account matching the invited email address.
The parameter accepts two values: True or False.
True-User must accept this invitation with firstname.lastname@example.org.
False– When a document is shared with an external user, email@example.com, it can be accepted by any user with access to the invitation link in the original e-mail.
Administrators who desire increased control over external collaborators should consider enabling this feature.