See if you have the same problem sending a message using yahoo webmail (a browser). There might be something wrong with the folder.

If that doesn't help try moving the most recent message in the sent folder to somewhere else, compact the account, and then send a message. If the problem goes away then it was probably due to something about the message you moved.

I do not have the same problem using yahoo webmail. I tried moving the message and compacting the account to no avail. When I create a whole new folder and SENT COPY and place a copy of sent e-mails to that folder via Account Settings it works. But, I can't get sent mail from the last few days to sync to either folder.

maybe it was saved in other place in your yahoo account