On the WP-SES forums, the developers said this is generally an issue with another plugin making a call to modify pluggable.php/wp_email() because their plugin does that.
I deactivated all other plugins except for woocommerce, and paypal for woocommerce first. Everything worked as expected. I enabled WP-SES and had the header issue.
I thought it might be Paypal Express because that’s the last user action before the error occurs.