Hi Zabelle,

This looks like an interesting issue and I suspect that you're actually looking in the wrong place.

Firstly, the moodle message table is used to store all notifications of any kind in Moodle. Once a message has either been sent by e-mail, or has been read (in the case of direct messages), the messaged is moved to the message_read table. This area is unlikely to be the cause of your issue.

The interesting thing here is that messages are delayed for groups of people.

What normally happens is that when cron runs, it:

1. determines what forum posts are new;
   
2. finds out the complete list of users who should receive a notification for the post;
3. marks each of the posts as having been sent (see the forum_posts table - it has a mailed column);
4. loops through each user and sends all of the messages belonging to that user;

When a message is sent in Moodle, it:

1. is turned into a message (in the message table);
2. is then processed by the list of message processors (e.g. e-mail, in-browser popup, jabber); and
3. is then moved from the message table to the message_read table.

You don't say whether the number of messages in your message_read table is growing, or whether it tends to be fairly static so it's difficult to know exactly what's going on here, but the other part of the puzzle is the Mail Transfer Agent (MTA).

When an e-mail is sent in Moodle it is typically passed to either an SMTP server, or a Mail Transfer Agent. In either case the messages are queued here while they are sent to the mail servers responsible for that e-mail address.

Often mail servers will be configured to restrict the amount of e-mail coming from a single server to prevent spam attacks. Sometimes this is done by only allowing the sender to send a few messages at a time, sometimes it is done by reducing the bandwidth available to send messages, and sometimes by temporarily rejecting messages and asking the sending server to try again later.

You'll often find that MTAs are also configured to reduce the amount of mail that they send at any one time when they're busy so as to make sure that they continue to be able to accept mail (e.g. from Moodle).

I suspect that you're encountering issues with your MTA - either that the recipient server is queueing messages somehow, or that it is queueing them because its load is increasing.

Hope this helps,

Andrew