From the Gmail Settings and section Forwarding and POP / IMAP, You can configure the account to automatically forward all mail to another address.
This can be very useful for users who manage multiple accounts the problem is that when setting up a referral does not show any warning on the main page (inbox) and this can be quite dangerous if someone else gains access to the account and configure it.
It may take weeks or even months without change is detected When was the last time to check that option? In addition to reading all the personal emails received, an attacker could access service users and passwords mailed that could even apply Reset password links for confirmation and other things that only the account holder should see.
This is not a new topic, many people have privacy issues with this option in Gmail, and some cases have been quite complicated to involve thefts from other accounts and services.
Checklist Gmail security:
Recently, there is a checklist that looks at 18 critical elements that could compromise the email account, it is worth taking a few minutes to complete (see here).
In one of the recommended just check the automatic forwarding settings, which should not do if the main page will display a warning when the filter first set up or modified:
Hotmail displays a warning:
On the other hand Microsoft webmail displays a warning on the home page announcing that the forwarding is activated, a message is annoying but at least this way if you set a forward without our consent as we notice easily. 
However, if the forwarding address amending the message is the same, but you can only set a direction, it may take several hours or days until the victim noticed that something is wrong.
It is worth mentioning that in order to access your account and modify not necessarily need to know the password, using programs like Firesheep could steal the session without any complications from another computer.
