I know this question is old, but I'm putting up an answer to help anybody who stumbles upon this..
I experienced the same and verify that this seems to be a problem with allowed size of attachments and how Outlook treats them. I moved a message with >20MB attachment to a temporary folder outside of 'Inbox' on Yahoo webmail and then Outlook was able to complete the send and receive process. Interestingly, the message with >20MB attachment had been downloaded successfully prior to this. Furthermore, the attachment was also downloaded successfully from Yahoo Webmail.