Hi ​@Olly01330 - Thanks for reaching out on this.

Unfortunately, this is something outside of our control as this fully depends on their email service/provider, so we are limited to what we can do on our end. In any case, your clients may find success by white listing our sending IP addresses at 167.89.22.99, 149.72.200.43, and 149.72.248.16 and making sure that notifications@calendly.com is on your safe senders list.

Outside of this, they would want to make sure they mark any emails from us as ‘Not Spam’ to ensure others don’t get caught in the same loop. Apologies there isn’t more we can do from our end!

Let me know if you have any other questions.

Calendly is listed with the IP (149.72.248.16) you mention as being blacklisted on SpamCop: 

https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a149.72.248.16&run=emailheaders

Ok, so what I’m understanding from this is there is nothing I can do?

There seems to be an issue with Calendly in this case. And no - we cannot fix it. They need to fix it asap. I have written an extensive support case to them, since mail deliverability has to be of the highest importance to them due to their business.