Throughout the years, virus scanners have proven over and over again to cause more issues than they claim to solve in Outlook. When you have a virus scanner installed which integrates itself with Outlook, it will scan outgoing emails 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. 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. This 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. How can I solve this and make sure Outlook only sends out one copy again? 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).

It doesn't happen with every email with attachments, just occasionally.

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

Stop duplicate emails in outlook 2013