If you suspect that your email account was hacked but cannot find and new emails in your sent folder, then the problem is worse than you thought and nothing to do with hacking into your account. I figured out how it was done awhile ago, but Yahoo won't do anything about it.
Here is how the hacker works the system:
1. Set up a sniffer on an email server.
2. Collect emails coming and going through that server.
3. Extract the header information. Over 99% of emails are transmitted using SMTP (Simple Mail Transport Protocol). You can actually read an email packet.
4. Extract the sender and receiver information.
5. Save this data for later.
6. Create a bogus email with links to sites with malware.
7. Use the previously collect sender and receiver information to complete the emails.
8. Insert the bogus emails into the email stream.
9. The bogus emails are treated like any other email by the mail server and delivered to unsuspecting victims.
Because the emails were usually sent between people that know each other the addresses are often in the other's friends list and are not filtered as junk.
The most common email server used for this attack is the Yahoo email server in Los Angeles, CA. Yahoo won't do anything without a court order because they fear that any action on their part may make them look culpable.
This explains why the emails do not show up in your sent folder, and also explains why changing your email password may have little affect.
This process has a name, "Spoofing". Spoofing is often used to make one node (computer, router or hub) appear as another node by stealing the other node's IP address, name and MAC address. It also applies to emails and phone calls claiming to be from someone that they are not.
Good luck getting your email address back.
