On the site I use the WP Mail SMTP plugin. I have that configured to send Order Confirmation emails and what not from a Gmail address dev@gmail.

Anytime an order comes in, dev@gmail will email a receipt etc.

I notice now that this email is sending approx. 15 emails a day in spam.

(Its only sending the email to one address, the address is actually the clients. So i setup a dev email for all my work and they have their own business one. All the spam is being sent to it and nothing else)

spam mail in sent box

A sent spam email

Does anyone know what might be causing this and how I could go about fixing it?

I notice one of the blog posts has an unusual amount of spam blocking in the comments, im not sure if that would have much to do with it.