Hi,
Which type of email account are you using?
This issue may be caused by several reasons. Please first check if the send/receive interval is set to a time that is too short. Outlook automatically check for new emails at a set interval. If the interval is set to a short time, the described problem may
happen. To check the setting, go to File > Options > Advanced > Send/Receive. We should set the automatic send/receive interval to more than 5 or even 10 minutes. The default interval is 30 minutes.
If above doesn't fix the issue, we may also check if there is any message stuck in the Outbox. If you can see a stuck email, just delete it. If you can't see anything in the Outbox, we may try to use the MFCMAPI tool to see if there is any hidden item in
the Outbox. To do this, please follow:
1. Download MFCMAPI tool and launch it.
2. Click Sessions > Logon, select your profile and click OK.
3. Double click your email account.
4. Right click on the top-level folder in the left panel and choose: Open contents table.
5. If you see an item with a subject prefixed with the Read:, select the message and choose: Actions-> Submit-> Abort submit.
6. Make sure the message is still selected and choose: Actions-> Delete message.
In addition, this issue may also happen if you have a virus scanner installed. To verify this, please try temporarily disable the virus scanner and then check if the issue will be gone.
Please let me know the result.
Regards,
Steve Fan
TechNet Community Support