Scheduling Follow-Up Emails Not Sending

Hello SS community.

This January I enabled follow up emails under Squarespace Scheduling. They seemed to have been sending up until recently, where customers have been emailing me saying they didn't receive the promised follow-up email containing a booking link.

I go under their profile and view the notification/emails sent, and from my end the emails are sending, but on their end they're not receiving.  (See screenshot -> all emails sent from us, last email in the screenshot not sent)

Squarespace support said it might be that these customers are marking our Scheduling emails as spam. However our customers know that they're expecting our emails with important follow-up info & links, so I highly doubt they're marking our emails as spam.

Is there a way to fix this?

Or is there an alternative way to set up follow-up emails for customers who booked a class?