Email Signature Management for Microsoft 365, Compared

Microsoft 365 has two admin-level ways to put one signature on everyone's outgoing email: an Exchange transport rule, or an Outlook add-in that an admin deploys. The rule adds the text on the mail server, in every client. The add-in shows the signature while people write, but only in Outlook versions that support it.

What each option does

Exchange transport rule. An admin creates a mail flow rule in the Exchange admin center. The rule appends or prepends HTML to each message as it passes through Exchange Online, whichever app sent it.

Outlook add-in. A small add-in runs when someone starts a new message, a reply or a forward. It inserts that person's signature into the compose window. The admin uploads the add-in once, and users receive it through the admin deployment.

Option 1: the transport rule

The steps are in our company-wide signature guide. This section covers what should change your decision.

• Tokens fill from the directory. Tokens such as %%DisplayName%%, %%Title%%, %%Phone%% and %%MobilePhone%% take their values from the sender's directory record. A blank field leaves a gap, so fill the directory before you roll out.
• Scope is every message by default. Microsoft's guide says the rule applies to incoming and outgoing messages unless you add a condition, such as "recipient is outside the organization".
• Replies get a second copy unless you add an exception. Microsoft's method is an exception that matches unique text in the signature, so the rule skips messages that already contain it.
• The default fallback can attach your mail. The fallback is Wrap. If the rule cannot insert the text, for example in an encrypted message, Wrap puts the original message inside a new one as an attachment. Microsoft advises against Wrap for rules that affect incoming mail from outside, because it interferes with Safe Attachments scanning, and suggests Reject instead.
• Senders do not see it while writing. The text is added in transit, so nobody sees a preview in the compose window.
• Test before you enforce. Choose Test without Policy Tips first. Microsoft says: "We recommend testing all mail flow rules prior to setting them to Enforce." Send a plain-text and an HTML test message to yourself, then forward one to check the reply exception.

Option 2: the admin-deployed add-in

• It needs an admin. Microsoft says event-based add-ins work only when an administrator deploys them. An add-in installed from the store does not start by itself.
• Upload path. In the Microsoft 365 admin center, go to Settings > Integrated apps, choose Upload custom apps, select Office Add-in as the app type for an add-in-only manifest, and upload the manifest file.
• Changes need consent again. If you change the manifest, users are blocked from the add-in until an admin accepts the change.
• It needs a connection. Microsoft says event-based add-ins require an internet connection to launch.
• Clients. Microsoft's event table lists OnNewMessageCompose for Outlook on the web, new Outlook for Windows, classic Outlook for Windows (version 1903 or later) and the new Outlook interface on Mac. The event covers new messages, replies and forwards, but not editing a draft.
• Some surfaces are skipped. Microsoft notes that activation may not work on some non-standard compose surfaces in Outlook on the web and the new Outlook for Windows.

Which one to choose

Start with three questions:

1. Do people need to see and edit the signature while writing? If yes, use the add-in. If the text can appear only in sent mail, the rule is simpler.
2. Does security object to a third party in the mail path? Neither option routes mail through a third party. The Sendoff add-in fetches signature text when someone starts a message. It does not send or relay mail.
3. Do you need it on phones? Test before you promise it. Mail sent from a phone still goes through Exchange Online, so a transport rule should apply, but check with a test message. The add-in does not cover phones (see below).

How Sendoff handles this

Sendoff builds one Outlook add-in per workspace. It is an XML manifest that listens for the new-message event, fetches the sender's Sendoff signature and inserts it with Office.js setSignatureAsync. An admin uploads that manifest as described above, and the signature is then added to new messages, replies and forwards in the clients listed above.

The manifest has no mobile entry, so iOS and Android do not get the signature automatically. Sendoff does not write Exchange transport rules. If you need a rule, use the steps in the company-wide signature guide. Microsoft 365 setup details are on the Microsoft 365 integration page.

Sources

• Microsoft Learn, "Organization-wide message disclaimers, signatures, footers, or headers in Exchange Online": https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/disclaimers-signatures-footers-or-headers
• Microsoft Learn, "Activate add-ins with events - Office Add-ins": https://learn.microsoft.com/en-us/office/dev/add-ins/develop/event-based-activation