Troubleshooting CardDAV fields

When using CardDAV to synchronize your contacts from Fastmail to another device or desktop application, you may notice that some fields don't match up, or that they are missing altogether.

The following information types are generally well defined, and should work with most CardDAV clients:

  • Names
  • Business names, departments, and/or positions
  • Birthdays
  • Email addresses
  • Phone numbers
  • Addresses

Newer information types are generally not as well defined, and so different clients may interpret or store the data in different ways. Where possible, we preserve whatever data we can. However, it is possible that there could be some data transformation or loss when editing these fields in the Fastmail web interface, compared with editing in a CardDAV client.

Why does this happen?

There is an agreed format for all services supporting CardDAV. However, this format has been through multiple revisions, and the fields change slightly with each revision. There are also optional extensions for extra fields, which only some clients support. There are also clients which store additional fields that are not part of any specification, and are unique to their platform only.

Fastmail never loses your data. Even if we are sent information we don't know anything about, we still store it even if we can't display it. This ensures we can continue to send all your data back to your device on the next sync.

What about import and export?

The vCard 3.0 format is what CardDAV sync uses, so that's going to be the most reliable to export and import of the available formats.

Fastmail also supports a number of other options for popular clients which we have matched up to their fields as closely as possible.

We strongly recommend checking that the import or export has successfully brought across all the information you need before deleting the original data.

Technical detail

Fastmail supports the CardDAV spec and uses the vCard 3.0 data format to transfer contact data.

While we do not display all fields, such as the X- private extensions, we still store the data.

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