This is an issue with the Yahoo! server. For some unknown reason it removes the draft flag on the message so you can’t edit it.
There is a workaround for this though. Go to Menu > Accounts and click on the Diagnostics tab for your Yahoo! account. In the Advanced Options Parameters box, paste in this text:
--imap-messages-in-drafts-folder-are-drafts
Click on Save & Close and restart eM Client. It will force all messages in the Drafts folder to be editable, no matter what the Yahoo! server says.
