CKB:Attachments are not stored in the Sent Items folder
Attachments are not stored in the sent items folder regardless of the size of the files set for "Do not save outgoing files if.." in the admininterface.
The issue is caused by the maximum allowed email size of the Exchange-Online server.
1. Login to Exchange Online admin center as the Global Admin
2. Click on recipients -> mailboxes
3. Click on the elipsis “…” and -> Set default message size restrictions
4. On the pop up windows set your Maximum Message Send and Recieve size (max. 150MB). Click OK to apply.
More informations on this topic: https://mspmagic.com/knowledge/settings-docs/exchange-online/set-maximum-message-size-default-setting-for-new-mailboxes/