For anyone else experiencing this issue:
I noticed that the sender email was set to: admin@mywebsite.com (mywebsite.com is a placeholder). I never set up an admin email, so I’m not sure why it was there.
To resolve this, I went to my domain host and created an email redirect for admin@mywebsite.com so that any emails sent to that address would be forwarded to me.
Then, I added admin@mywebsite.com as a verified sender in SendGrid.