Why are messages sometimes received multiple times?

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Question

Sometimes users receive multiple copies of the same message. This seems to happen mostly when a mail is sent to a large number of users at the same time, but can happen under other circumstances as well. What is the cause of this?

Answer

If a client is used to send a message to a large number of recipients, (such as 100 or more) it can often take longer than usual for the server to process the message, since far more work is involved than when sending to a single address.

Many mail clients only wait for one minute before giving up when sending a message, and so disconnect. When this happens the server is still processing the message and delivers it correctly.

Because the client has given up waiting, it does not know that this message has been sent, and so retries. This can then result in the message being delivered many times.

Waiting for only one minute is usually fine, however in this case you are asking the client to perform the function that is usually performed by a list server, such as GMS Communication Server.

Sending to many hundreds of addresses is a task that is best performed by a list server, however if you do require this from your client, you will need to ensure that the client timeout (that is the length of time the client waits for a response from the server) is increased to a suitable value.

Determining a suitable value depends on the performance of your server hardware – you can view the GMS log files to see how long message processing is taking.

Other circumstances when multiple messages might be sent even if there are not a large number of recipients are when a message is very large or requires a lot of processing. This will occur if you have complicated GMS Anti-Spam filters set up, or your server is slow at processing messages because of hardware processing issues.

One final reason why this may be happening is that the user forwards copies of all incoming messages on to another account, or there is another account on the system which is an alias of this user. That other account has reached its quota, thus when GMS attempts to write the message to this second mailbox it is unable to do so. This results in GMS returning a temporary failure message to the sending server, which then proceeds to resend the message again.

The solution to this is to bring the second mailbox back within quota or rectify the problem that is stopping mail being delivered to the mailbox.

See Also:

Keywords:multiple copies message

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