One of our customers uses the bulk emailer (newsletter) functionality of DNN quite a bit.  He had a very old version of the software (4.x).  About 2 months ago, the bulk emailer stopped sending to all recipients.  He was no longer getting the confirmation email and checking with his clients, many people were not receiving the emails.  We looked into it and could not find any errors in the DNN logs or the server logs.  We upgraded the app to the latest version of DNN (7.x) and had him try again.  He got the same results.  I did a test by sending a bulk email just to myself with no issue.  Again, there are no log events that lead to the issue.  My guess is that IIS is killing the thread that is processing the bulk emails after a certain period and it is silently dying but I have not looked into this yet.

Any help or pointers to the issue would be greatly appreciated.

By the way - He is sending to about 1400 recipients and the last email was very simple (just text).

Are all of the emails sent at one time? It could also be that the email provider your client is using has restricted the number of emails that can be sent per hour or day.  If that is the case you will need to use an email module that can throttle the email to limit the number of emails sent within a specific period of time.

Check the DNN log files in "\Portals\_default\Logs", the DNN Event Viewer and the servers Application Event Log for any exceptions that indicate the site is restarting during a run.

Do you allow developers to modify the files in production directly or do you use staging servers?  If developers are changing the files directly on the production server that trigger recompiles and they are doing it often then it could be that the max recompile limit has been met and the site is restarting.  The default is 15.  You would see this indicated in the servers Event Log.

Do you have multiple sites sharing the same application pool?

Do you know if the bulk emailer runs directly from the module or is it executed from the DNN Scheduler?