Q: Sent emails disappear no spam button

I have OS X Yosemite 10.10.3 on a new mac book air early 2015, missing sent emails, I have yahoo mail, also spam button is grayed over does not work? I do know that my email did go through but I have no record of it.  As a temporary fix I send myself a bc of any emails that are very important to me,  this doesn't happen to every email only happens randomly.  However the spam thumbs down is grayed over and does not work.