For the client who was having trouble with 492 mailouts, I never really got to see if it was timing out or not (I did not have access to her error logs to check on the cron job), but I did add the
set_time_limit(0) piece and some debugging to see if it was completing the recipient mailout loop.
It was. She was using gmail for SMTP with WP-Mail-SMTP plugin, and her sent items only showed around 150. I switched it to Mailgun for testing, hardcoded a test address in bbpress-notify-nospam, and it worked. Mind you, this was for regular notifications, not the Digest, but the fix should be the same.
I’m completing some additions for the Digest plugin and will add the time limit code to it. I haven’t used DNSExit, so I don’t know if it shows you the number of mailouts, but I recommend that you check it for dropped messages.