I do remember in the past, when I started having issues with email been rejected, obviously we got a spike on tickets because this problem was affecting the whole company. After checking the NDR (Non-delivery report) and different workarounds, I realize that a certificate for exchange has expired.
There are different approaches to identify whether a certificate is about to expire; in this article https://tonygonzalez0379.com/2020/05/25/how-to-check-certificates-expiration-date-using-powershell, you can find how to use PowerShell to identify proactively if a certificate will expire in a period of time, in my case, I use 30 days in advance, so this script is being executed on weekends. You have plenty of time to renew the certificate before you significantly impact all end users.
Another common issue that affects the email flow is when a domain name is about to expire; sometimes, we get a domain name only for a year, and then when this domain expires, Exchange can’t continue delivering email to those users that contain the specific domain in the email address.
It is common that a user has different SMTP addresses, because those accounts need to receive emails using other domains, the same approach as the certificate, you could create a PowerShell script to validate the expiration date using The Who is information for those domains.
The good news is that Microsoft 365 now has a solution that can help the administrators to proactively identify if a certificate or domain name that is part of the registered domains in M365 is about to expire; this feature is new, and you will be able to see this notification in the insight area.
Also, the administrator will receive an email with this notification. I believe that this is great for all the messaging administrators in the company, because if you are not proactive, there is a high risk of having a problem like everyone can’t send or receive emails, and the business impact could be huge.
Below you can see an example of the domain expiration. You can find this in the exchange ministration console on Office 365, in the inside tab. In the same way, you will find the information about the certificates here. Nonetheless, it is better to have a solution like PowerShell, this is the old school, and pretty much all the Administrators are familiar with this process.
Thanks for reading.