...
Global default values for specific email related settings
Max Events per email - How many events an email should contain, an event is one action, e.g. one ISSUE_CREATE and ISSUE_COMMENTED equal 2 events.
...
Max seconds to wait when Blocked [5] - The is a fail safe option designed to abandon being blocked if the wait is too long (in seconds). If the wait is abandoned then the email will not be queue, or sent. Instead the email will be left to a future queue flush.
Digest Exclusions
This section is repeated at the Mail Queue and User Profile screen. If the JQL any of these screen matches then the event information will be sent immediately. If any of them have "Also Digest Immediate Sends" then the event will also be digested.
Exclusion JQL - This options allows you to specify a JQL statement that will be used to determine in the notification of an event should be sent immediately, rather than digesting.
...