One day when we login our Gmail, I found there was an Alarm in red color on the top:
Warning: We believe your account was recently accessed from: xxxx (inetia.pl: xx.xxx.xxx.xxx).
We knew our Gmail password might be stolen, or there was hacker from some where accessed our Gmail account.
When we click the detail link, we found there were not only one time the other people accessed our Gmail:
The first step we should do is change password immediately.
And then, check is there other place your Gmail is using: Scroll down your Gmail to the bottom, you can see there is information on the top bottom corner indicates how many place current Gmail is using and the login time.
If you found the Gmail is open in other places, please click the Details link and Sign out all other sessions:
Change all other settings about security, see as following:
Change Recovery settings, the security question you set for your account:
Also, you have to double check whether your Gmail was set other stuff such as forward automatically, multiple log in etc. So go to Gmail settings to check:
Check each setting item carefully, for example: whether your signature was input some robot script, remove the signature content then; and Forwarding, POP and IMAP, you’d better disable all of them if you can not make sure you have set something by yourself.