Hi @Pier-Olivier L Cabana ,
Welcome to our forum!
Considering that i don't have the same account as yours, so I tested it with a Gmail account instead. When i configure Gmail as IMAP in Outlook for Mac, the Sent folder of Gmail can be synced successfully. In order to further configure if the issue is related to your account, please try to configure other account(such as Outlook.com, Gmail) as IMAP in your Outlook for Mac to check if there're any differences.
If the Sent folder of other accounts configured as IMAP can be synced, I'm afraid that the issue may be related to your current account. If the same is true for other accounts, please try the suggestions mentioned in this thread for troubleshooting: Outlook for Mac sync issues.
Hope to help you!
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