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.