Beware: User Information List (UIL) acts as memory for PeoplePicker when need to renew Azure AD B2B guest account for changed email

In Azure AD B2B, guest accounts are provisioned and redeemed with as their unique external account identifier, the own external email address of the invited guests. In rare situation that the external email changes, the guest account situation needs to be synchronized with this external identity change. However, as the email address is the primary identifier, it is not possible to change this as attribute in the existing Azure AD guest account. One needs to create in the Azure AD of the inviting company a new guest account based on the new / modified external email address, and potentially can also delete the no longer valid old guest account. On the SharePoint Online side, in each site collection in which the guest needs access, the access must also be repeated for the new guest account. Something to be aware of in such situation is the relationship between SharePoint PeoplePicker and the hidden User Information List (UIL) in site collection. The PeoplePicker uses UIL as one of its entrances to try to resolve user information for looked-up user name. As of SharePoint 2013, whenever an account on individual basis is granted access to a SharePoint site collection, immediate an entry for that account is administrated in the UIL (in SharePoint 2010 and before, the addition in UIL was delayed until first access by the authorized account). The effect in the use case where guest account is renewed by deleting from and creating new, is that the PeoplePicker default will still resolve to the user entry in UIL that was created there when earlier the guest was authorized on the previous guest account. Resulting in an inconsistent situation: the guest is assigned a new Azure AD guest account, the former might even be deleted in Azure AD, but in SharePoint Online via UIL as memory the previous account is selected by PeoplePicker upon granting access. Effect is that the guest can successful logon with the renewed guest account against the Azure AD of inviting company, but in the next step this authenticated guest account is not granted access in the SharePoint Online site: "You need permission to access this site". The way to resolve this is to go as Site Collection Administrator (SCA) into the UIL through <site-url>/_layouts/15/people.aspx?MembershipGroupId=0 , select the entry that refers to the previous guest account, and in the 'Actions' menu select "Delete users from Site Collection". After this corrective management action, the PeoplePicker on looking up the guest (by name or email) will no longer find the stale entry in the UIL, and create a new one that corresponds correct with the renewed guest account. And the invited Azure AD guest is enabled to successful access the site via modified email address as his/her 'bring-your-own-identity'.