It might not be hacked. I had a similar problem before (I take it you have lots of messages that couldn't be delivered and it was those email that alerted you) but when I checked my sent emails there was nothing weird there. I got in touch with yahoo (it was my yahoo account) and they said they people can make it look like they've sent an email from your account but they haven't really Here is some of what yahoo wrote to me after I reported it:
Hello,
Thank you for writing to Yahoo! Mail.
It appears that the sender of this message forged the header
information
to give the impression that it came from your email address. The sender
seems to have used your email address in the "reply-to" and/or the
"from" field of the message sent out and, as a result, misdirected
email
is being returned to you.
When you receive mail from a "MAILER-DAEMON" or a "Mail Delivery
Subsystem", with a subject similar to "Failed Delivery", this means
that
a sent message was undeliverable and has been "bounced" back to you.
These messages are produced automatically by the intended recipients
email service and usually include a reason for the delivery failure.
Bounced messages normally consist of two parts; the first contains the
reason for the bounce, the second contains the original message.
