Sharing with an external user who has changed their email

SharePoint is not matching addresses by textual similarity or domain ownership. It resolves an identity object—usually a Microsoft Entra B2B guest—and caches it in the site’s User Information List. An alias, changed mail property, or stale cache can display the old address while targeting the same object.
Ask an Entra administrator to locate the guest and verify its object ID, mail, otherMails, and identities instead of creating a duplicate. If the person must sign in with the new address, update the email and use Reset redemption status. Microsoft documents that this preserves the guest’s object ID, group memberships, and app assignments while sending a new invitation.
After redemption, test sharing by entering the full new address. If SharePoint still resolves the obsolete value, an administrator can remove the stale site-level UserInfo entry following Microsoft guidance, then resolve the guest again. Audit links first so access is not accidentally lost.