As per the description provided, the mails are not saved in the sent item folder of one of the email account.
If you have setup an account as IMAP account type in Outlook then follow the steps mentioned below:
1. Open Outlook > Click on File tab > Account Settings.
2. With the account Settings dialog open, select the account and click ‘Change’ > click on ‘More Settings’.
3. Click on the ‘Sent Items’ tab > Ensure you have checked the option ‘Save sent items in the following folder on the server:’ and choose the appropriate folder where the ‘Sent Items’ needs to be saved.
Hope the above information helps. For further queries, reply and we will be happy to help you.
Thank you.