I've saved a search and set up an alert to run the search every 10 minutes and send out email notification if the number of result is greater then 0, it was working fine at the start (about 5 months ago) after some time it only sent out the email 7 out of 10 times and for the last 2 days I haven't got any email but I can see that the alert was triggered a couple of times in the trigger history. What could be causing this issue? (Splunk 6.3)
in your alert is there an attachement? if there is an attachement verify if it exceeds you email limits: test if your alert runs without attachements.
You can verify this also viewing splunkd.log and searching for email error messages.