Emails are not saved to Sent Items – Outlook | Microsoft Docs

The Save copies of messages in the Sent Items folder option in Outlook is disabled. Resolution. Enable the Save copies of messages in the Sent Items folder option by following the steps below for your version of Outlook. In Outlook 2010 or later versions: Select File, and then Options. In the Outlook Options dialog, select Mail.

https://docs.microsoft.com/en-us/outlook/troubleshoot/email-management/email-sent-are-not-saved-to-sent-items-folder