I recently upgraded from 2.5 to 2.7. I am getting reports that you can no longer send messages to all participants. I am able to duplicate the problem with a test course with only two participants. If I select just one participant I can send a message. If I select both participants I will receive the following error.
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Error calling message processor email

line 231 of /lib/messagelib.php: call to debugging()
line 2185 of /message/lib.php: call to message_send()
line 158 of /user/messageselect.php: call to message_post_message()

Something went wrong while messaging selected users. Some may have received the email.
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This error has been mentioned quite a bit but there are no applicable solutions for me. It was reported fixed in 2.0 but again it is just now showing up for us in 2.7. It is not a SMTP issue since I can send individual messages and forum posts are emailing out fine. It's something specific to Selecting All Participants --> Send Message.

That's about all I have to go on.

Thanks,
Bryan