Take a look: https://moodle.org/mod/forum/discuss.php?d=388490
It seems that it means the messages are not being really sent... they are getting queued... (???)
This is the log of a sent message:
Adhoc task complete: mod_forum\task\send_user_notifications
Execute adhoc task: mod_forum\task\send_user_notifications
... started 15:39:05. Current memory use 47.4MB.
Sending messages to xxxxxx (XX_ID_XX)
Post 14066 sent
Sent 1 messages with 0 failures
It seems that it means the messages are not being really sent... they are getting queued... (???)
This is the log of a sent message:
Adhoc task complete: mod_forum\task\send_user_notifications
Execute adhoc task: mod_forum\task\send_user_notifications
... started 15:39:05. Current memory use 47.4MB.
Sending messages to xxxxxx (XX_ID_XX)
Post 14066 sent
Sent 1 messages with 0 failures