Only 2 users are complaining that, they are not getting daily alert emails from one sub site (others are getting alerts). these 2 users are getting alerts emails from different sub sites from same site collection. We checked exchange server log, Yes exchange server not have their email ids. That means that SharePoint not sending emails to them.
Why they are getting alerts from different sub sites not from one particular site? How can I debug and solve this?
How can set log for alert emails in central admin? So I can check users emails are going or not.
I've put together a SharePoint alert troubleshooting guide - some problems can be hard to resolve but hope it helps.
If you're successfully receiving alerts from other sites then you can ignore the steps for checking your email infrastructure is setup correctly (relay allowed etc).