If you are having issues with actions not syncing across webmail, you may want to make sure that the account is set up as an IMAP account. You may also find the following article helpful:
I'm having the same problem, and the only help I ever seem to find suggested anywhere is "make sure its set up as an IMAP account"
BUT it is! Mail automaticly sets all yahoo accounts up as IMAP. So is this just a yahoo problem? Is there anyway to fix this?
For me the bigger issue isnt that I need to be able to check my e-mail through a web browser regulerly, but on my Iphone and Ipad the account, which is through the mail app, also dosnt show sent mail sent through the mac app.
I agree this makes mail on my mac totally usless.
I finally found this..
I tested and already works.. it is kind of lame that we need to do this extra config for something otherwise simple task
hope it works for you