Emails being unintentionally resent by Outlook

Issues

Lately, I developed a Python tool that automates sending emails through the Outlook client. After a while, I noticed that sometimes the same email is sent twice. I went through my code thoroughly but couldn’t identify any factors that could be causing this issue. After some debugging, I realized that the duplicated emails were always sent right after the moment Outlook was opened…

Solution

I came across a post¹ incidentally that explained the reason behind the duplicate email sending behavior in the Outlook client. It turns out that if the Outlook client doesn’t receive a reply from the Exchange server after sending the first email, it sends a second one to ensure the delivery.

My Python tool was closing the Outlook client shortly after sending the email, which I suspected might be preventing it from receiving a reply from the Exchange server. To resolve this issue, I added a 30-second waiting period before closing the Outlook client, and the problem disappeared.

If you have any questions or advice, leave me a comment below and I will try my best to respond!