Apple Mail extensibility (MailKit)
SithBit ships integrations for the hosts that let a third party extend them — the Thunderbird extension, the Outlook add-in, and the Chrome extension. Apple Mail is the most-used mail client by a wide margin, so it is worth recording exactly what its extension surface allows, and why there is no Apple Mail client here today.
The supported path: MailKit
Since macOS 12 (Monterey), the only sanctioned way to extend Apple Mail is a MailKit app extension — a bundle shipped inside an ordinary macOS app that Mail loads out of process over XPC, so it cannot crash Mail or read its internals. The surface is exactly four extension points, and nothing more:
| Extension point | What it can do |
|---|---|
MEComposeSessionHandler | Inspect and annotate an outgoing message; add headers; block the send |
MEMessageActionHandler | Act on an incoming message — move, flag, set a colour |
MEContentBlocker | Block remote content (scripts, styles, images) in the message view |
MEMessageSecurityHandler | Sign, encrypt, and decrypt message bodies — the S/MIME-style crypto hook |
The old mail bundles — undocumented plug-ins that loaded straight into Mail’s own process and could do essentially anything — were removed entirely in macOS 14 (Sonoma). MailKit is now the only path, on macOS.
Why there is no Apple Mail client (yet)
The interesting hook for SithBit is MEMessageSecurityHandler: it is the
defined place to plug in a custom decryption scheme, so a MailKit extension
could in principle unseal a SithBit sealed body inline as
Mail renders it. Two constraints keep it off the near-term roadmap:
- macOS only. There is no MailKit on iOS or iPadOS — the Apple client that dominates the mobile open statistics is not extensible at all. The iPhone story stays the standalone webmail app, not a Mail extension.
- No arbitrary UI. A MailKit extension gets the four points above, not the
free rein the old bundles had. SithBit’s other clients share one rich
account-management pane (
webclients/shared/); Apple Mail cannot host that pane, so it would be a separate, much thinner integration — decrypt-on-display and little else.
In short. A read-only “unseal my SithBit mail in Apple Mail on the Mac” extension is technically possible via
MEMessageSecurityHandler; a full-featured Apple Mail client, or anything on iPhone/iPad, is not.
See Apple’s MailKit documentation for the framework reference and Build Mail app extensions for the extension-point walkthrough.