Hi @frob ,
21 before the expiration date, Site Owners will receive an email warning them about the soon to be expired guests. They will also see a similar warning message on their SharePoint site warning them that the guest user is about to expire. By clicking Manage they will be able to extend access.
As the picture shows:
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Update
If site admin has full control to site, he should receive it as a site owner. If site is only created by site admin and does not have the permissions assigned to himself for site, it will not be received. In other words, those who have full control permissions on site will be alerted.
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