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Outlook sending multiple copies of email

I’m having a problem with Outlook sending multiple copies of some emails – usually those with attachments.

It doesn’t happen with every email with attachments, just occasionally. It will send it 20-30 times but keeps the email in my Outbox saying that it hasn’t been sent (although sometimes it says it’s been sent but stays in the Outbox).

How can I solve this and make sure Outlook only sends out one copy again?

Sending Duplicates buttonThis often happens when you are using a virus scanner that integrates with Outlook or when your Send/Receive interval is set to a very short time.

In some cases, especially when you have a slow connection to your mail host, extending the mail server time-out may solve it as well.

Virus scanner integration

When you have a virus scanner installed which integrates itself with Outlook, it will scan outgoing emails as well.

Throughout the years, virus scanners have proven over and over again to cause more issues than they claim to solve in Outlook.

It is really recommended to disable or uninstall your virus scanner’s Outlook integration capabilities. For steps on how to properly disable the integration see the documentation for your virus scanner.

For more background information see “Duplicate E-mails” and “Disable virus scanner integration?“.

Short Send/Receive interval

When your send/receive interval is set to a very short period (below 5 minutes) and your message takes a while to upload to the mail server, it could be that the next send/receive interval already started before the previous one completes.

This will then cause a backlog of Send/Receive tasks which still need to be completed. In the worst case scenario, it could be that the message still in the Outbox will get resubmitted for sending and thus create a duplicate.

To change your Send/Receive interval go to:

  • Outlook 2007 and previous:
    Tools-> Options…-> tab: Mail Setup-> Send/Receive…
  • Outlook 2010 and Outlook 2013
    File-> Options-> Advanced-> Send/Receive…

The default value is 30 minutes. As mentioned before, don’t set it lower than 5 minutes. When you have 3 or more accounts configured, it is recommended not to set it lower than 10 minutes.

Server Timeouts

POP3 and IMAP accounts work with Server Timeouts. This basically tells Outlook when to consider a connection as “lost” when no responds has been received after a configured amount of time.

By default, this is 1 minute. When you are on a slow or unreliable connection or a connection with a high latency (like mobile connections), then it could be that you reach this timeout. In that case, Outlook will resubmit any message in the Outbox upon the next Send/Receive interval.

When the message was actually already submitted to the mail server but the acknowledgement was not received before the timeout was reached, the message will be submitted again and thus create a duplicate as well.

Increasing the Server Timeout to 2 or 3 minutes may solve your issue. Directly setting it longer than 3 minutes is not recommended as it often means that the actual issue lies somewhere else.

Increase the Server Timeout when you have a slow or unreliable connection.
Increase the Server Timeout when you have a slow or unreliable connection.