Use your Microsoft 365 mailbox in a better client. Without leaving Microsoft

Outlook on Microsoft 365 is a fine email service. The mailbox is fast, the spam filtering is competent, the calendar is the one your colleagues already use. This page is not about leaving it. It’s about the client you read it through, especially if you also keep a personal Gmail, a Fastmail, or a ProtonMail next to your work inbox.

Microsoft 365 is the service. Epistles is a client

A great deal of confusion in this category comes from the word “Outlook” meaning two different things. There is Outlook the mailbox, run by Microsoft on their Exchange Online infrastructure, where your mail actually lives. And there is Outlook the application, the software that talks to that mailbox, in any of its flavours: Outlook for Windows, the new Outlook for Mac, Outlook on the web, the Outlook mobile apps, and the various third-party clients that connect through Microsoft’s APIs.

Epistles is the second kind of thing. Your mail stays at Microsoft. Your calendar stays at Microsoft. Your contacts, your filters, your shared mailboxes, your retention policies, your administrator, all of that stays exactly where it is. Epistles is the window you read through. Connecting an Outlook account to Epistles changes the application; it does not migrate, copy, or move your mail anywhere. You can disconnect tomorrow and Outlook on the web is still there, untouched.

The same window Outlook uses

When you load Outlook on the web in your browser, the page underneath the chrome is talking to a Microsoft endpoint at outlook.office.com/owa/service.svc, posting JSON envelopes typed as FindItem:#Exchange, UpdateItem:#Exchange, GetCalendarView:#Exchange. That is the API Outlook on the web is. It is not documented for third parties; it is what Microsoft built for their own client; it has every feature Microsoft 365 supports because Microsoft’s own client is the thing using it.

Epistles speaks that same API. Our Microsoft adapter posts the same JSON shapes to the same endpoint, with OAuth tokens scoped to outlook.office.com, the way Outlook on the web does. The categories you set, the flags, the conversation structure, the calendar invitations, the rules: everything is there because we are calling the surface those features were designed against in the first place.

We did not pick this path because it is fashionable. We picked it because the alternatives lose. Microsoft Graph, the public API Microsoft markets to third parties, is a reduced surface with a different audience claim and a slower release cadence; for a mail client that wants parity with what Outlook itself can do, it is the wrong tool. EWS is the legacy SOAP surface for on-premises Exchange and is on its way out for Microsoft 365 anyway. Both are answers to a different question. Outlook on the web’s own protocol is the one with the features.

Real-time push, properly

Microsoft 365 supports real-time push through a SignalR channel called the OWA notification channel, the same one their own Outlook web client uses. We connect to it directly, so a new message appears in Epistles within a second of arriving at the server, with no polling and no wake-on-schedule pattern draining your battery.

The complication is that Microsoft has been migrating tenants from a legacy ASP.NET Framework SignalR transport to a newer ASP.NET Core one, and a given Microsoft 365 organisation can be on either generation. Epistles detects which one the tenant runs from the negotiation response and adapts to both, including the handshake and token-passing differences. Most third-party Outlook clients skip push entirely and poll every few minutes instead, which is why their mail feels a step behind. We thought it was worth doing properly.

One client, every account

The reason we built Epistles, and the reason a Microsoft 365 user might prefer it over Outlook itself, is that almost nobody only has an Outlook account. The typical working life involves a work Microsoft 365 mailbox alongside a personal Gmail that has carried the family logistics for a decade, a Fastmail address that has outlasted three jobs, sometimes a ProtonMail for the political work, and a generic IMAP account that’s been on the same domain since 2003.

Outlook can connect to some of these. Microsoft has been quietly improving Outlook’s ability to add a Gmail account, and the new Outlook for Mac will take an iCloud address. The result is functional but not generous: every account that isn’t a Microsoft account in Outlook is treated as second-class. Categories disappear, labels become folders, the search runs against Microsoft’s server-side index of mail it had to copy from Google to make work at all, your Gmail filters live in one place and your Outlook rules in another. You can use it. It does not feel like home for the non-Microsoft half of your inbox.

Epistles takes a different approach. Each provider speaks its own native protocol. Microsoft 365 through Outlook’s own private API. Gmail through the Gmail API, with labels staying labels. Fastmail through JMAP. ProtonMail through Proton’s own API with OpenPGP decryption on your device. Apple Mail / iCloud through a single app-specific-password wizard that provisions IMAP for mail and CalDAV/CardDAV for iCloud calendar and contacts at the same time. IMAP and SMTP for everything else. Each adapter is loaded only when you use it, so a Microsoft-and-Gmail user never downloads ProtonMail’s crypto code, and a mailbox that doesn’t use IMAP doesn’t pay for it.

The effect is one application, one settings panel, one keyboard map, with every account speaking the dialect it was meant to speak. Your Outlook calendar shows up alongside your Gmail labels and your ProtonMail’s encrypted threads, in a single window, on every device you read mail on.

How Epistles compares to other Outlook clients

Outlook on the web

Microsoft’s own browser client, the one Epistles’ Microsoft adapter shares an API with. Excellent for a single Microsoft 365 account, increasingly less so for two, no story at all for a non-Microsoft account next to it. Lives in a tab, not in your dock.

The new Outlook for Windows and Mac

Microsoft’s desktop replacement for the classic Outlook. Built on web technology, ties tightly to Microsoft 365, can add Gmail and iCloud through a server-side mirroring layer that makes them feel like guests. Memory footprint is widely complained about; not Microsoft’s priority to fix.

Classic Outlook for Windows

Decades of features, a deep integration with Office, and a reputation for being heavy. Microsoft has stated it is on a glide path to retirement; the new Outlook is the future they are pointing customers towards.

Apple Mail with Exchange

Native, fast, free, on macOS and iOS only. Connects to Microsoft 365 through Microsoft’s Exchange ActiveSync flavour, a separate protocol from the one Outlook on the web speaks and an increasingly limited one. Calendar invitations and categories work; advanced features and shared mailboxes do not.

Third-party clients on Graph or IMAP

Most third-party clients reach Microsoft 365 either through the public Graph API, which is a reduced feature surface, or by flattening the mailbox to IMAP, which loses categories, conversations, the calendar, and most of what makes Microsoft 365 Microsoft 365. Worth asking which path your current client takes.

Epistles

Outlook’s own private API throughout, the JSON service.svc surface. Real-time push via the OWA SignalR channel, both generations supported. macOS, Linux, Windows, iOS, Android, and a web app at app.epistles.com all shipping today, with Apple Watch and Wear OS apps for triage on the wrist. Microsoft 365 next to Gmail, Fastmail, ProtonMail, Apple Mail / iCloud (single ASP wizard for mail + iCloud calendar + iCloud contacts), and IMAP, each in its native protocol.

One watch story, both ecosystems

Outlook ships an Apple Watch app and a Wear OS app, and on this axis they are the only mainstream third-party Outlook client that does. We match that coverage. Epistles ships an Apple Watch app for triage on the wrist (archive, snooze, voice or template reply, “Reply & Done,” pin, forward to contact, agenda with RSVP, plus WidgetKit complications) and a Wear OS app with the same feature set for Galaxy Watch, Pixel Watch, and any Wear OS 3+ device, including an inbox tile. The Wear OS app in particular is rare in this market: most third-party clients (Mimestream, Spark, Mailbird, Canary, Superhuman, Thunderbird) ship neither watch platform. Outlook ships both, we ship both, and the rest are catching up.

What you give up

We are an email client, not an Outlook clone, and there are parts of the Microsoft ecosystem we don’t replicate. The honest list:

• Teams integration. Outlook’s deep ties to Teams chats, meetings, and presence are not in Epistles, and we have no plan to embed Teams. If you live in Teams, you will still want it open in its own application.
• Advanced calendar features. Epistles reads and writes Outlook calendar events through the same private API, including invitations, recurring events, and scheduling. Shared calendars and delegated mailboxes work. Resource booking, scheduling assistant with free-busy across rooms, and Microsoft Bookings are things Outlook still does better.
• Microsoft 365 administrator features. Mail flow rules, compliance holds, eDiscovery, message tracing: those are administrator surfaces and stay in the Microsoft 365 admin centre where they belong. Epistles is a user’s client.
• Shared mailboxes and public folders are partially supported. Personal shared mailboxes work; the older Exchange public folders, which use a tree separate from regular mail, are limited.
• Copilot and Microsoft’s AI writing tools are not in Epistles and won’t be. We do not ship AI features that route your mail through a model provider; we don’t want that responsibility, and we don’t want your mail in those logs.

Privacy

Your Microsoft 365 mail travels directly between your device and Microsoft over Microsoft’s own API; our servers are not in that path. We do not see message bodies, subjects, recipients, or attachments. OAuth refresh tokens are the one carve-out: those live on our backend so we can hand each device a short-lived access token without the chain races that bricked older multi-device setups, and they are encrypted at rest with a server-held key. Everything else, including the rest of your account record, is in the Cloud Vault as opaque ciphertext encrypted under a key derived from your Epistles password, which we don’t hold. The security page has the longer version.

If your inbox is the shape this page describes

Microsoft 365 is the mailbox you have. Epistles is one of the better windows you can read it through, and it is the one that also lets your other accounts in, each in its own dialect, on every device you keep them on. Fifteen-day trial, no card up front, three accounts on Free afterwards. Join the waitlist, or read the rest of the site.