Hi @ssali colline,
Thank you for posting in this community.
Since there are many reasons why an email may fail to send, please follow the steps below to test and narrow down the cause of the problem:
1.Please have your users check to see if the shared email has been placed in the junk mailbox, sometimes emails are sent successfully but are incorrectly considered as junk mails.
2.Are all users not receiving emails? Please try sharing for a single user. Generally, when you share the content in SharePoint with group email, the email notification will be sent to the group mailbox in Outlook. The individual user in this group will not receive the mail notification in their personal mailboxes.
3.Whether all SharePoint libraries have the same problem? You could create a new SharePoint library to check.
4.Check if the same issue occurs with other SharePoint site collections.
5.Please try restarting the IIS service and then see if the problem persists.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. It will be beneficial to more community members reading here.
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