Mail not forwarded at the same time for many users

Question

Some of my users remark that they did not received the same mail at the same time. Causing many "Didn't you read the mail?" situation...

What explain that the same mail is not forwarded to the users at least in the same 15 minutes?

Here's a log example of what I mean :

15:54:10   Object: TEST   dst:[e-mail]   src: [e-mail]
15:54:10   Object: TEST   dst:[e-mail]   src: [e-mail]
15:54:09   Object: TEST   dst:[e-mail]   src: [e-mail]
15:54:09   Object: TEST   dst:[e-mail]   src: [e-mail]
15:14:09   Object: TEST   dst:[e-mail]   src: [e-mail]
14:54:09   Object: TEST   dst:[e-mail]   src: [e-mail]
14:43:18   Object: TEST   dst:[e-mail]   src: [e-mail]
14:43:12   Object: TEST   dst:[e-mail]   src: [e-mail]

Answer 1

What explain that the same mail is not forwarded to the users at least in the same 15 minutes?

My guess would be that you have a large number of people you are forwarding mail to on a relatively busy network or server.

Mail servers often have delays when there is an inability to conduct a transaction. When a transaction cannot be conducted, many mail servers will wait some period of time (which may or may not be configurable) before retrying to send any given mail item. This could happen multiple times in a row.

Likewise, mail servers often perform bulk mailing in batches. One thing to remember about batch mailing is that there may be an automatic delay between batches to prevent the mailings from being treated as spam or otherwise overwhelming the receiver. Again this delay may or may not be configurable.

The upshot is to examine the server and your network to spot bottlenecks which could be hindering effective sending of your mail. But be aware there may be delays you simply can't adjust for.

As a small aside, it feels to me like a common misconception that email is instantaneous. It isn't, at least not always. It may not be up to you, but if you want a method of distributing information effectively and instantly, either a centralized web page with updated content or instant messaging with something like Openfire can be a good route.

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Let's Play A Game...

As thought experiment with the data you posted, lets imagine a situation where one batch cannot be started until the last is completed:

Batch 1

1 ) User 8 and 7's are responsive due to low network traffic and the transactions complete quickly:

    14:43:12   Object: TEST   dst:[e-mail]   src: [e-mail]
    14:43:18   Object: TEST   dst:[e-mail]   src: [e-mail]

2 ) Unfortunately, there is a spike in traffic shortly after the first two items are sent and throw off User 6's transaction. The mail server decides to wait 5 minutes to retry but again fails at 14:49. So it waits another 5 minutes and the transaction is finally successful (10 minute delay):

    14:54:09   Object: TEST   dst:[e-mail]   src: [e-mail]

3 ) The congestion has only gotten worse. Now there are 4 transaction failures at 5 minute retry intervals for User 5's transaction (20 minutes delay):

    15:14:09   Object: TEST   dst:[e-mail]   src: [e-mail]

Batch 2

4 ) Batch 1 has finished. Based on some settings, the server decides to wait 10 minutes before starting to process Batch 2 at 15:24.

The mail server tries to conduct a transaction for User 4's email, the first one in Batch 2. Unfortunately, the continuing traffic spike has made successful transactions relatively impossible, resulting in another 30 minute delay (6 failed attempts). This results in a total of 40 minutes before the first email in Batch 2 can be sent:

   15:54:09   Object: TEST   dst:[e-mail]   src: [e-mail]

5 ) Thankfully, the busy period is over and the mail server can now complete the rest of the Batch 2 transactions quickly:

   15:54:09   Object: TEST   dst:[e-mail]   src: [e-mail]

   15:54:10   Object: TEST   dst:[e-mail]   src: [e-mail]

   15:54:10   Object: TEST   dst:[e-mail]   src: [e-mail]