I had a similar issue - emails stopped sending even though no email settings were changed. After a good bit of frustration, I stumbled on an error message in the cron log that said something about the time zone setting. Evidently, setting the time zone to UTC-X (any number) somehow caused the email to fail. Once I changed the timezone setting back to 'server's local time' it immediately started sending the emails again.
I just been battling this issue as well.
To fix this I did
chown administrator:www-data -R mod/forum
Then I opened up the mdl_task_scheduled table and added in a row for the forum
component - classname
mod_forum - \mod_forum\task\cron_task
I'm running the latest Moodle 3.1, and this issue was present with the last release 3.0.4.
Crons are running correctly
All the mailbox settings are checked and have been modified to see if there is a difference
ISP has checked all the server side mailbox settings
I've created the following mailboxes > inbox.tobeconfirmed@domain and tobeconfirmed@domain
I'm now looking for a wall to bang my head on! Any help would be gratefully received!!
Running Cron again gets this report:
>>
Execute scheduled task: Forum mailings and maintenance jobs (mod_forum\task\cron_task)
... started 09:44:10. Current memory use 11.8MB.
Processing user 4
Processing user 13
Sending Error: lib/moodlelib.php email_to_user(): SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
post 790: Re: Test message
Processing user 1437
Processing user 6
Processing user 19
Sending Error: lib/moodlelib.php email_to_user(): SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
post 790: Re: Test message
Processing user 3
Processing user 2
Processing user 24
Sending Error: lib/moodlelib.php email_to_user(): SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
post 790: Re: Test message
3 users were sent post 790, 'Re: Test message'
Starting digest processing...
Cleaned old digest records
... used 72 dbqueries
... used 1.0838561058044 seconds
Scheduled task complete: Forum mailings and maintenance jobs (mod_forum\task\cron_task)
Execute scheduled task: Incoming email pickup (tool_messageinbound\task\pickup_task)
... started 09:44:11. Current memory use 20.9MB.
Connecting to mail.s702.sureserver.com as moodle.2016@domain
Connection established.
Searching for Unseen, Unflagged email in the folder 'INBOX'
Found 0 messages to parse. Parsing...
... used 0 dbqueries
... used 0.06174898147583 seconds
Scheduled task complete: Incoming email pickup (tool_messageinbound\task\pickup_task)
Execute scheduled task: Removes any unnecessary event monitor events (tool_monitor\task\clean_events)
... started 09:44:11. Current memory use 22.4MB.