Sent emails are not being saved....

Hi,

Based on your sharing, if the messages remain initially and then disappear after roughly one day, this is more likely caused by a mailbox retention policy or server-side rule. Therefore, I suggest you should first check the policy applied to Sent Items:

• Right-click Sent Items > select Properties > Policy tab.
• Check Folder Policy and Online Archive settings. Look for anything like:
  • 1 Day Delete
  • Delete after 1 day
  • Move to Archive
  • Use Parent Folder Policy

If the issue still persists, the best next step is to contact your company's IT department. Organizations can apply retention policies specifically to default folders such as Sent Items, and those policies can delete or archive messages even when Outlook is configured to save sent copies. Most importantly, required organizational policies cannot be removed by user.

You can ask your administrator to check the mailbox’s Sent Items retention tag, assigned MRM/Purview retention policies, server-side inbox rules, Online Archive, and mailbox audit activity for move or delete operations. Your admin can also help you recover the deleted messages if they're still in Recoverable Items or the second-stage Recycle Bin.

In case you do not know who your IT admin is, kindly refer to this article: How do I find my Microsoft 365 admin? - Microsoft Support  

I hope this information is helpful. Should you have any further questions or need additional assistance, please feel free to share them in the comments below. I'm very happy to help.