I also just remembered this.
I think your using sendinblue for a outbound. I believe i remember discussing that with you.
They are now Brevo.
Sendinblue/Brevo has logs in the transactional section of all emails sent. Should show if the message was delivered, read and i believe it also know if a mail was replied to.
EDIT: Actually no. no replies tracked as a replys go though their outbound. But at least you will know if the message was received and if it was opened.
EDIT: And i should also point out that you need to log into your account there at least once every 3 months. I found out the hard way that they suspend accounts that are not logged into on a regular basis.