In that case, that is a default Gmail mark letting you know it’s not sure of the sender. If it was spam, the mark would be an exclamation mark, and so on. It has nothing to do with WooCommerce or the email templates.
Kindly be aware that neither WordPress nor WooCommerce sends the emails. What happens is WooCommerce calls the wp_mail() function, which then requests WordPress to send the email. Because WordPress is not an email server, it typically asks PHP to send the email for it. PHP then checks for a local email server within the webserver and tells that email server to send the email.
You can read here more about how does email work with WordPress:
https://woocommerce.com/document/email-faq/#h-how-does-email-work-with-wordpress
If you find it pertinent, you can set up an SMTP provider to deliver your emails instead of your server, following the instructions outlined here:
https://woocommerce.com/document/email-faq/#section-5
I hope this helps!