Requirements
What you need
Section titled “What you need”Sigil requires Microsoft 365 with Exchange Online. The add-in is an Office.js add-in and the personalisation reads Microsoft Graph, so both halves are Microsoft-native and neither works elsewhere.
You need an account that can do two things:
| Task | Permission needed |
|---|---|
| Grant admin consent to Sigil’s Entra application | Privileged Role Administrator or Global Administrator |
| Upload an add-in under Integrated apps | Global Administrator |
In practice one Global Administrator covers both. The consent step installs Sigil’s service principal in your tenant and grants the read-only Graph permissions listed in permissions.
No servers, connectors or mail flow rules are involved. Sigil never sits in your mail flow.
Client support
Section titled “Client support”| Client | Automatic on compose | “My signature” pane |
|---|---|---|
| Outlook on Windows (classic) | Yes | Yes |
| Outlook on Windows (new) | Yes | Yes |
| Outlook on the web | Yes | Yes |
| Outlook on Mac | Yes | Yes |
| Outlook for iOS | Yes | No |
| Outlook for Android | Yes | No |
Outlook mobile activates add-ins in read mode only. Event-based activation is one of a small number of documented exceptions, but a compose task pane is not, so the manifest declares events and nothing else for the mobile form factor.
Mobile therefore has no manual fallback. Somebody whose signature fails on a phone recovers by opening the pane once on a desktop client, because consent is per user rather than per device.
Two mobile behaviours worth knowing: on a reply the signature is not visible
until the compose window is expanded to full screen, and OnNewMessageCompose
does not fire for a message started from the iOS Share sheet.
Domains Sigil uses
Section titled “Domains Sigil uses”| Host | Purpose |
|---|---|
portal.usesigil.app |
Admin portal and the signature API |
static.usesigil.app |
The add-in bundle and its manifest |
e-clk.usesigil.app |
Tracked link redirects only |
If your organisation filters outbound traffic, allow all three. The add-in
fetches from portal.usesigil.app and is itself served from
static.usesigil.app. The link domain answers /r/ redirects and returns 404
for every other path, so it carries no API or portal surface.
One host Sigil does not own is also involved, and it is not specific to Sigil:
| Host | Purpose | If it is blocked |
|---|---|---|
appsforoffice.microsoft.com |
Office.js, which Microsoft requires every Office add-in to load from here | The add-in does not run at all |
Office.js is loaded by both the automatic path and the pane, so blocking it stops signatures rather than degrading them. If you already permit Office add-ins, it is allowed, because no add-in works without it.
Nothing else needs allowing. The icons in the “My signature” pane were once
fetched from unpkg.com; they now ship inside the add-in bundle, so an
organisation that blocks general-purpose content delivery networks loses nothing
at all.
The typefaces the pane and the admin portal are set in are served from Sigil’s
own hosts, static.usesigil.app for the pane and portal.usesigil.app for the
portal. No font host needs allowing, and neither fonts.googleapis.com nor
fonts.gstatic.com is contacted. If an older allow-list still names them, or
still names unpkg.com, those entries can be removed.
Template constraints
Section titled “Template constraints”Outlook itself sets limits on what a signature can contain. Design within them from the start rather than discovering them at publish time.
A rendered signature must stay under 30,000 characters. The portal blocks a publish that would exceed it.
SVG is not supported. Use PNG or JPG.
Images are referenced as cid: and attached inline by the add-in. A hosted
<img src="https://..."> would be suppressed by Outlook’s default image blocking
and render as a broken image.
Outlook constraints explains each of these in more detail.
Only one event-based add-in runs at a time
Section titled “Only one event-based add-in runs at a time”If your organisation has other event-based add-ins deployed, they run sequentially rather than in parallel. This rarely causes a problem, but it is worth knowing if signatures appear more slowly than expected in a tenant with several such add-ins.