I found out yesterday when a friend texted me saying he was getting spam from my address.
I logged into my account and found some failed delivery messages of emails I'd never sent myself.
There is no trace of sent emails left in the Sent Mail folder.
I also can't see any suspicious login activity in my gmail login history at the time the spam was sent or any other time.
The thing that's worrying is that they got hold of my contacts. It was an account I was not using directly any more, so I have closed that account now.
I have 2-factor auth enabled on my other accounts and have even changed all the passwords again.
I have reported twice to Google, but haven't heard back anything yet.
Edited to add:
I have looked at the original email message and the header does suggest it came from google. I reported the Message ID to google along with the full email message.
I also suspected it may be some app or service that I gave access to my contacts to, but I didn't see any in my app access list.
That's why I'm turning to you guys to ask if any of you've been in a similar situation, how do I secure my accounts, and trace down the cause of this particular incident?
Thanks