I don’t receive notifications from notifications@calendly.com. As it turns out, Calendly IP Adresses are publicly blacklisted. You can see it here: https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a167.89.22.99&run=toolpage
Calendly is the one who is sending emails from IP adress: 167.89.22.99
OrgName: SendGrid, Inc.
OrgId: SENDG-12
Address: Twilio, Inc.
Address: 1801 California Street
Address: Suite 500
You should be able to delist it from blacklist
https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a167.89.22.99&run=toolpage
If you're not be able to do this, please try use clear IP adress, which is not listed of any kind of blacklist.