To test the limits in a recent moodle install, I prepared batches of users to be uploaded in increasing size progression. There were 3 of them, of sizes 65 / 160 / 402, for a total of 627 registered users. Without any further intervention, 121 users entered the course succesfully.
After some reports came in that showed that probably some of the password generation notification mails that moodle sends had not been received, an email was sent on our part prompting the 506 users to enter the course or report if they had not received the e-mail.
It is kind of a MOOC situation, so we are not concerned by non-responders, but 140 users got back saying that they hadn't received the confirmation e-mail. Due to spam filters and multiple inboxes, we cannot trust the users as to if they really got the e-mail. In fact, 15 of them entered the course on their own after having said they hadn't received the email.
By further interaction with users we confirmed that at least in some cases it was true that no mail was sent. We generated a list of all our users and see if they appear in this curated list of potential errors. When the batches are split, it becomes clear that the second and third batch effectively hit a bottleneck, and at some point, these emails were discarded.
1. This is a plain Moodle 3.8.2 installation, all emailing set to defaults so far. Where can I read about how this PHP based (I guess?) moodle email system works ?
2. What measures could be taken to minimize the risk of moodle notification of registration e-mail be classified as spam ?
3. What are the possible bottlenecks that could have been hit in my situation ? (Uploading users, generate password and send email) What logs can I read to see where the error was ?
Thanks in advance !!