This sometimes 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.
Resetting your modem, router, hub, WiFi Access Point and/or other network appliances may sometimes already be the answer too even when all other network connections seem OK.
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. This will not put you at risk in any way. For steps on how to properly disable the integration, see the documentation for your virus scanner.
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.
You can open the Send/Receive Progress dialog to see if there is a backlog:
- Outlook 2007 and previous
Tools-> Send/Receive-> Send/Receive Settings-> Show Progress…
- Outlook 2010 and Outlook 2013
tab View-> button: Show Progress
You should only see up to 2 tasks per mail account in this dialog. When an account is listed more than that, your send/receive interval is too short.
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.
Checking for new emails too often could interfere with the sending process.
For Outlook 2010, Outlook 2013 and Outlook 2016
Outlook 2010 and later do remember which emails have been downloaded before as it stores this information in the pst-file itself.
Upon configuring your account, you can specify the delivery location to an already existing pst-file instead of letting Outlook create a new one for you. If you have retrieved your account settings via AutoDiscover, you can select the option to manually configure your account. This will get you to a screen where the account settings retrieved via AutoDiscover are shown and also has a section “Deliver new message to” to select your original pst-file.
To change the delivery pst-file for your account after you have configured your account use;
File-> Account Settings-> Account Settings-> select your e-mail account-> button Change Folder-> select New Outlook Data File… and browse to your original pst-file.
Synchronization Tools are great to keep your contacts you have in Outlook and on your mobile device, like a tablet or smartphone, synchronized and up-to-date. Depending on the tool you are using, synchronizing for the first time might be tricky and could cause some duplicates or “similar” items.
For instance, I used to store most of my mobile phone contacts by first name only and the ones in Outlook by their full name. So I had a contact named Edwin on my mobile phone and a contact named Edwin Sparnaaij (yep, trying to get my brother famous here :-D) in Outlook. When I would synchronize, I would end up with an item named Edwin and an item named Edwin Sparnaaij in both Outlook and my mobile phone since the synch tool doesn’t know these are one and the same person in real life.
To prevent this from happening, make sure your contacts are in order before synchronizing for the first time or you’ll only duplicate the mess. Since my mobile phone was a big mess mainly because of the lack of a proper input device and contact storage properties on it (I had a Nokia 3210 at that time) getting things in order would be handier in Outlook. To make sure it doesn’t mix with your original Outlook contacts you can create a new mail profile first with a dummy account or no account configured at all (tip: call the profile Mobile Device so you can easily recognize it). Now you can safely synchronize with that mail profile and use Outlook to easily clean up your contacts on your mobile.
If you are a control freak (I won’t be last to admit this) and want to make sure that the initial synchronization indeed doesn’t create any duplicates, move the cleaned up contacts from the Mobile Device mail profile to your original one;
- Start Outlook with your original mail profile.
- Connect to the pst-file from the Mobile Device profile by File-> Open-> Outlook Data File…
- Move the Contacts from the Mobile Device pst-file to your original Contacts folder.
Now that you have created a “master copy” in Outlook make a back-up of your pst-file in case synchronization goes wrong after all. Also now that we have all contacts correctly in Outlook, we don’t want it to still end up creating duplicates because the synchronization tool isn’t “intelligent” enough to recognize the similar items (this depends on the synchronization tool you are using so I’m going for a fail safe here). As we have a master copy in Outlook we can safely delete all the contacts on the mobile device. Now reconfigure your synchronization tool to use your original mail profile and synchronize; there is just NO WAY you can end up with duplicates now!
POP3 and IMAP accounts work with Server Timeouts. This basically tells Outlook when to consider a connection as “lost” when no server 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.