In the time of configuring schedule report, ensure that roles selected do not have large number of users because the mail server restricts the number of recipients and this might impact the other reports.
Check the server.log for keyword with AutoReportSend (This is the name of the class that is responsible for sending). If you find any exceptions, see if you can find any SMTP code such as the below:
Sending has failed with the below message:
552 5.3.4 Message size exceeds fixed maximum message size
Using such SMTP codes or exception messages we can identify what might be causing the sending failed.
If you come across “Too Many Recipients” exception being thrown repeatedly in the server.log then one of the scheduled reports would be configured to send the report to User role where more number of users are mapped.
This can be checked by searching for reports by using below query:
select scopevalue from autoreportmaster_details where scope=1
Here scopevalue represents the SD Project Roles which can be checked if it is inherited from submitter
also take the autoreportid and check if it is Non ITSM report in which case you need to check if the scopevalue is mapped to groupid user from groupdetail table