Hi,
If the alert mails are not received, then you'd like another mail been sent automatically, right?
After the first mail been sent, the users will receive the mail definitely if there is not any issue with your Outlook clients. So I guess what you want to know is that whether they have read the message rather than received the message.
In this situation, it is suggested to ask them to send a message indicating that they know it after they receive your alert mail.
If this is not your case, please provide more details about your issue.
Regards,
Aidan Wang
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