It is in my gmail "sent" folder.
Every other mail there is "hidden@gmail.com" as sender.
The spam mail has "hidden@gmail.com via sendgrid.me" as the sender, in my "sent" folder.
Gmail has in help page about via:
I see "via" and a website name next to the sender's name
You'll see "via" and a website name next to the sender's name if the domain it was sent from doesn't match the domain in the "From:" address. For example, you got an email from
john.smith@gmail.com, but it could've been sent through a social networking site and not Gmail.
You can't remove the "via" next to someone's name. Gmail shows this information so you're aware of where your messages are coming from.
If you notice that an email was sent via a program you don't recognize, the message might be spam.