Hi @KayNet ,
Welcome to Q&A forum.
How did you check the Sent Items folder? Via VBA method as well? If so, in order to conduct further researches on your issue, it is suggested for you to manually check your Sent Items folder in Outlook desktop client and web mail to see if the issue has any difference.
By the way, can this issue be reproduced if you directly send emails from Outlook desktop client or web mail? If it can work fine when sending from Outlook desktop client or web mail, I'm afraid that your issue might be more related to your VBA method. Please kindly understand that here we mainly focus on general issues about Outlook desktop client and know little about develop issues. I did some search about issue, still could not found any useful information. So, to better solve your issue, it is suggested for you to post a new thread to the dedicated forum for Outlook for developers.
https://social.msdn.microsoft.com/Forums/office/en-US/home?forum=outlookdev
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thanks for your understanding and hope your question will be resolved soon.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.