Although users did set alerts on library or folders, they were not able to receive any alerts. Basically outgoing mail failed from the SharePoint server.

One of the errors was : “A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provide”

I followed the following MS documentations to resolve the issue:

1. Make sure Outgoing E-Mail settings in SP CA is set as in step 6 of the following article. https://docs.microsoft.com/en-us/archive/blogs/mikelee/configure-tls-encryption-between-sharepoint-on-premise-and-exchange
2. Also make sure you have created a receive connector in your Exchange server as show in step 7. https://docs.microsoft.com/en-us/archive/blogs/mikelee/configure-tls-encryption-between-sharepoint-on-premise-and-exchange
3. Test sending email form PS, using the following PS script. https://docs.microsoft.com/en-us/archive/blogs/mikelee/test-email-from-sharepoint-using-powershell
4. Follow the steps 28 to the end of the article .https://docs.microsoft.com/en-us/archive/blogs/mikelee/configure-tls-encryption-between-sharepoint-on-premise-and-exchange
5. Use the exchange server FQDN on the outbound SMTP server option in Outgoing E-Mail settings instead of aliases like mail.abc.com.
6. Make sure you add Exchange server and CA root certificates into Manage Trust option under CA->Security->Manage Trust.