Hi @Phuong Kuhn ,
I understand that you get this error "The total attachment size exceeds the limit. Try removing one or more attachments before saving or sending." in your Outlook desktop client, is it right?
If so, are you working with Outlook for mac? And could you send message normally in your web mail or other mail client?
Based on my research, this error may occur when the total attachment size is too large, and I found some relevant threads that mentioned search large attachment in your mailbox and delete them may work for it. If you haven't tried this way, it's suggested to follow these steps to delete the large attachment to test if your issue has any difference after that:
Select your mailbox, tap on the Outlook Search bar
The search ribbon opens up
Click on attachments >greater than 10 MB
This will open up a search bar. In that change the size from 10000 to 30000 (depending on your attachment size)
In the search ribbon change the selection to "All Mail" - 3rd button from top left in the ribbon or 'All Items' to get the results from all the Outlook items.
Right click on the message, click Attachments and Remove All.
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