Cross Domain Sharing


Ever since the early days of FastMail, we've offered the ability to share email folders between users. When we added Calendar and Contact access via CalDAV and CardDAV, the ability to share between users in your family or
was a key feature of our platform.

Until now, you have only been able to share with users on the same domain - for example or your own personal domain. If you had a business with multiple domains, your options were limited by the contraints of our system.

From today, all users are equal - you can share with everybody in your family or business, no matter what domain they use.

If you are interested in the technical details behind this change, read on!

NOTE: due to the limitations with the current service, all '.' characters in domains and folders will appear as '^' via IMAP. They appear correctly in our web interface.

Cyrus IMAP virtual domains

Under the hood, we're using the Cyrus IMAP server. Until recently, it didn't support sharing across domains at all. You may have read about one of the steps during our advent blog series last
. Having a fast reverse-ACL lookup allows us to search the entire mailbox list quickly.

This is important, because the mailbox list is sorted by domain first. The format of a mailbox in the mailboxes.db file is like this!user.brong.Trash

So the LIST command could use a prefix search for! to reduce the number of records loaded. This kept things relatively fast even on busy servers.

For horrible legacy reasons, Cyrus supports two different types of virtual domains. We use the one called userid - where domains are based on the username you log in with rather than the IP address you connect on. If you're an admin, you could always see users in other domains, but it used a non-standard way to display the mailboxes, so my listing would look like:

No normal mail client would cope with that. You can see why it was chosen though, the alternative of is totally ambiguous.

Unix Hierarchy Separator

There are two other switches within Cyrus that change LIST behaviour, the booleans altnamespace and unixhierarchysep. If you use altnamespace, the names of your folders are transplanted up a level, while shared folders and other users are batched together in specially named folders.

It's easiest to explain with some examples. I'll do them without domains for simplicity:

Internal Format:

altnamespace: false / unixhierarchysep: false


altnamespace: false / unixhierarchysep: true


altnamespace: true / unixhierarchysep: false

Other Users.aaearly
Other Users.friend
Shared Folders.zzlate

altnamespace: true / unixhierarchysep: true

Other Users/aaearly
Other Users/friend
Other Users/friend/foo
Shared Folders/shared/news
Shared Folders/zzlate

You'll notice some things there about sorting order of the three namespaces (INBOX, user, shared) - that the shared folders have no prefix at all (just not user). An exciting number of things to keep right when making any changes, and I've been working hard to simplify these parts and place them behind reliable interfaces for a while now.

That pesky dot

Like many things in the IMAP standard, the hierarchy delimiter was a compromise between the netnews style dot and the filesystem style slash. One of the reasons for unixhierarchysep was to allow dots in user names, so I could have for example:


Nice, but how do we store that internally? The answer was to choose a character which was not common in mailbox names, and disallow it - then map the name. So internally you would see:


A way to represent domains

One nice thing about this namespace is that if you allow '@' in names as well, you can have cross domain support! In fact, if you turn off virtdomain support entirely, you can get that:


And that will correctly render as:


But we had virtdomains turned on already, and we needed to keep backwards compatibility.

Exposing the internals

I thought this was all going to be impossible, but then I realised that I could take advantage of that internal representation. In the non-unixhierarchysep world I could just translate all dots in the individual elements and the result would be consistent, and look good when we switch to unixhierarchysep in the future.

After that, it was a simple matter of a ton of programming and writing test cases to make sure everything else still works!

Some of these internals are still visible. If you log in via IMAP you will see funny looking names. We do the translation in our web interface to keep the folder names and usernames pretty, but we won't have those via IMAP until we switch on unixhierarchysep for users, which is coming with the security and multi factor authentication changes that are planned for later this year.

So here's what it looks like:

* OK [CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE XAPPLEPUSHSERVICE AUTH=PLAIN AUTH=LOGIN SASL-IR] sloti30t15 Cyrus IMAP 3.0.0-beta1-git-future-12410 server ready
. login ...
. LIST "" *
* LIST (\HasChildren) "." INBOX
* LIST (\HasNoChildren) "." INBOX.AICSA
* LIST (\HasChildren \Archive) "." INBOX.Archive
* LIST (\HasNoChildren) "." INBOX.uidtest6
* LIST (\HasNoChildren) "." user.test2
* LIST (\HasNoChildren) "." user.testcrossdom@gondwana^name.Archive
. OK Completed (0.020 secs 78 calls)

(yes, I own

Again because of backwards compatibility, you can see that's INBOX has no domain, because that's how FastMail sharing already works. I had to revert the "always show domain" behaviour which is preferred for consistency.

Show me the code

The main commits are:

The end result is simpler for users to understand and consistent. When we have an IMAP mode that uses unix hierarchy separators, it will be very nice to use. There's still work to do on making Alt Namespace better, but I'll write about that another day.