Our institution is using Moodle for some years. Moodle was the secondary LMS tool up to september 2016 with about 600 courses and we never encountered email delivery problems before. Since september 2016, Moodle has become the first LMS and the number of courses increased to reach today more than 2600 courses. And since some months, we remark delays in mail delivery coming from Moodle.
We followed specifically one news sending in a course implying 440 students. 150 students received the message on the first day, 30 of them received the message on the second day, ... an one week later still 20 students did not yet receive the message.
We had a look in the database and found that all messages are stored in the Moodle "messages" table, with one line for one email to send to one person. We also observed that the lines in this table are deleted once the email has been sent.
Our Moodle is a 2.9.4+ and we use the default mail sending method using PHP.
Cron is runned every 15 minutes.
We found a lot of post forums of people having problems with mail delivery but with Moodle that do not send any message.
This did not help us to understant the reason for delay.
It would be very interresting for us to understand what happens with messages in the "messages" table and specifically when they are deleted.
Thanks in advance for informations and suggestions.