I do not know what might be problem.
But, remove call to APEX_MAIL.PUSH_QUEUE_BACKGROUND procedure and see does it make any difference.
That procedure is not documented and you should not use it.
I have commented out the call to PUSH_QUEUE_BACKGROUND, but problem still exists.
Emails are sent when I run the UTL_SMTP package, but not with APEX_MAIL package. I was assuming APEX_MAIL is just a wrapper around UTL_SMTP Package but it doesn't appear to be the case. As a result all our IR report subscriptions are stuck in the Mail queue and not being delivered at all. As I've mentioned before the issue is Only with one of the Database instances, it works fine in other but I've the problem just with one instance.
I verified all the Instance settings logging in as Apex Administrator but really can't figure out what the issue here is. Appreciate any help !!
Did you set up the network ACLs correctly?