[codingfounder]

The header does suggest it came from google.

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.

How does one get hold of the address book without hacking into the account or via a third party app that has access?

[bdfh42]

You will probably be amazed at the number of web sites and phone apps that have had access to your contact list - just to get something done. Easy to forget when and when.

Could even be that it was someone you know who has your email address who gave their contact list to this spammer (directly or indirectly). The bad guys could then have simply picked one email address as the from and got to it spamming the rest.

[fjarlq]

Post the headers (after scratching out the email usernames)... there may be clues lurking.

[codingfounder]

I'm trying not to leak any more personal info, so I'm not sure which parts of the message I can post publicly.