This error message is most often caused by having an email with an attachment that's too large to send stuck in your outbox.
Try deleting the big email from your outbox and sharing the attachment using a service such as Google Drive or Outlook OneDrive.
It has been reported that these steps work too:
1. Go to Outlook and run an Advanced Find. (Click in the search box and a new menu “Search” will open – you will see the button for Search Tools and then Advanced Find).
2. In the new window “Advanced Find” – on the first line, click browse, and check all the folders (and subfolders) in the email account (You might be able to click the Search Subfolders dialog box, but it didn’t seem to work for me so I checked all the boxes). Hit OK
3. Go to the second tab “More Choices” and only click the radio box that says “Only items with one or more attachments”. Do NOT click or add anything else.
4. Click Find Now (on the right).
5. When all the emails have been displayed, sort by SIZE. You are only interested in the emails that show up with a size of 0 (ZERO!!!!!). Open the emails and you will probably see that these emails have attachments that add up to over 20MB. Delete them – and then delete them again from the trash bin.
6. Close outlook and reopen it. Run Send/Receive All Folders and it should run without the error message.
