Sync Kolab is an add-on for Thunderbird that synchronizes your address book as well as calendar data. The calendar features are optional and only available if you use the Lightning add-on (I do not). You can store your address book data in an IMAP folder or on a Kolab server. I’ve long been an IMAP user and I do not wish to administer a Kolab server so I’ve opted for the IMAP backend.
I’ve been using Sync Kolab for months now but I’ve always been disappointed. Thunderbird has an option to choose the format of messages, either plain text or HTML, for each contact. This is great because I like the richer formatting available in HTML but HTML isn’t appropriate for many situations. Mailing lists are a prime example of when to use plain text. I’d finally grown annoyed enough that I started peering into the source code to add the feature myself.
What I found in the code was that the feature was already supported — if you are using the VCARD backend. Now that I’ve switched to the VCARD backend all is well!