Gmail – Why Gmail sent has an unknown Yahoo account accompanying it

See additionalemail@gmail.com is an alias for realemail@gmail.com, which implies that addtionalemail@gmail.com is something like: real.e.mail@gmail.com re.alemail@gmail.com or realemail+anything@gmail.com

This is not a separate GMail account and so you cannot receive separate emails for this. You do not have a separate inbox for this. You can create a filter so that the emails sent to the additionalemail go to a particular label.

You can try creating accounts on different sites using additionalemail, you try to subscribe to something using this additional email.

If you go to GMail>Settings>Accounts and Imports you can add additionalemail under the Send Mail as and send e-mails as additionalemail (but the complete header of the sent email would reveal your realemail@gmail.com).

You could be in one of the following situations:

• you could have a keylogger/trojan on your computer;
• you could have logged in from a computer that had a keylogger/trojan installed;
• you could have been a phishing scam victim;
• you could have used a service that required your gmail account password (less likely) and they either store it in plain text and got hacked or were just rogue and sold/used it.

Now, you should:

• change all the passwords that you received on your gmail account, for each service/server;
• get in touch with any financial organization (e.g. banks) that might have sent you sensitive info on this email - if that's the case, follow bank account and credit card movements.

And last, you should switch to 2-step verification for your gmail account. Details are on gmail blog: http://googleblog.blogspot.com/2011/02/advanced-sign-in-security-for-your.html

Also, make sure your computer is clean - no viruses, trojans, keyloggers.

Edit:

It's also possible that you have the password stored in plain text and that file got somehow misplaced (trojan, file sharing error).