I had to think to recall the reason. I had a support ticket at Fastmail about this in 2024 - I wanted to host my own CardDAV server and have Fastmail use and sync to this. This was not possible - I was told that two-way sync with other services was a "calendar only feature". I might re-visit.
Comments
Is there a reason why you don’t use FastMail’s CalDav server ?
https://www.fastmail.help/hc/en-us/articles/1500000278342-Se...
And CardDAV too:
https://www.fastmail.com/blog/carddav-your-contacts-everywhe...
I had to think to recall the reason. I had a support ticket at Fastmail about this in 2024 - I wanted to host my own CardDAV server and have Fastmail use and sync to this. This was not possible - I was told that two-way sync with other services was a "calendar only feature". I might re-visit.