An Exchange Online mail flow rule signature is a rule that adds the same HTML or plain text to the top or bottom of mail as it enters or leaves your organization. You build it in the Exchange admin center under Mail flow, Rules, Add a rule, Apply disclaimers. These steps follow Microsoft Learn’s page on disclaimers, signatures, footers or headers, its overview of mail flow rules and the Microsoft 365 admin page on organization-wide signatures, all read in October 2026. This is general information for admins, not legal advice.
Exchange online mail flow rule signature in short
- Mail flow, Rules, Add a rule, Apply disclaimers.
- Choose which messages: outside recipients, or all messages.
- append a disclaimer (bottom) or prepend a disclaimer (top), then Enter text.
- Pick a fallback option, add an exception so the text is not repeated, and test before you enforce.
- Know the limits: not under the latest reply, not visible in Sent Items.
What an Exchange Online mail flow rule signature is
Microsoft explains that mail flow rules (also called transport rules) take action on messages while they are in transit, not after delivery to the mailbox. That is why the result is a server-side stamp. The text is added after the sender presses send, which has consequences you will meet below.
The admin page also notes that organization-wide signatures are called disclaimers, whatever they contain. So the action you look for is Apply disclaimers, even if all you want is a name and a phone line. Individual users can still add their own signatures in Outlook or Outlook on the web, and the rule is a second layer on top. For the personal side, see adding a signature in Outlook on the web.
Build the Exchange Online mail flow rule signature in the admin center
Go to the Exchange admin center and open Mail flow, then Rules. Select Add a rule, then Apply disclaimers. You work through three wizard pages.
Page 1: Set rule conditions
| Setting | What to choose | Why |
|---|---|---|
| Name | Something you will recognise later | Rule lists grow |
| Apply this rule if | The recipient is external, then Outside the organization, or Apply to all messages | Mail flow rules apply to inbound and outbound mail by default, so a condition keeps the text off internal mail |
| Do the following | Apply a disclaimer to the message, then append a disclaimer or prepend a disclaimer | Append puts it at the bottom, prepend at the top |
| Enter text | Your HTML or plain text | See the snippets below |
| Select one | The fallback option | Explained below |
| Except if | The subject or body, then Subject or body matches these text patterns, and enter a phrase from your text | Stops the text being added again on every reply |
Microsoft’s guidance is to add an exception that looks for unique text in the disclaimer, so it is added only to the original message in a conversation. Choose a phrase that appears nowhere else in your mail.
Page 2: Set rule settings
- Rule mode: Enforce switches it on at once. Microsoft also offers test modes that log results without adding the text, and recommends testing every mail flow rule before enforcing it.
- Severity: the level shown in the message log.
- Activate this rule on and Deactivate this rule on: an optional date range. A seasonal banner can switch itself off.
- Stop processing more rules: stops later rules acting on the message.
- Defer the message if rule processing doesn’t complete: by default the rule is ignored and delivery continues.
- Match sender address in message: Header is the default. The other values are Envelope and Header or envelope.
- Comments: a note for whoever inherits the rule.
Page 3: Review and finish
Check the summary and select Finish. Microsoft’s overview says a new or changed rule can take up to 30 minutes to apply, so do not judge the first test too quickly.
HTML snippets for an Exchange Online mail flow rule signature
Microsoft says the disclaimer text can include HTML tags and inline CSS, and that images can be added with the image tag. It also lists tokens that take values from the sender. We have not run these in a tenant, so treat them as starting points and test each one.
| Purpose | Snippet |
|---|---|
| Small legal line | |
| Sender details from the directory | |
| A phone line | |
| A hosted logo | |
The tokens come from Microsoft’s list, which includes %%DisplayName%%, %%Title%%, %%Phone%%, %%MobilePhone%%, %%Company%% and %%Office%%. Put each token on its own line. The admin page says rules skip a line that contains a variable they cannot fill, so a person with no phone number loses only the phone line, not the whole block.
On images the two pages differ. The Exchange page allows the image tag and shows a logo hosted at a web address. The admin page, by contrast, says images cannot be embedded. Reading them together, a hosted image is possible and an embedded one is not. Mail apps that block remote pictures will not show the logo until the reader allows it.
The fallback option for an Exchange Online mail flow rule signature
Some messages cannot be changed, such as an encrypted one. The fallback tells Exchange what to do then.
| Option | What happens | Microsoft’s caution |
|---|---|---|
| Wrap | A new message is created, the original goes in as an attachment, and the text is added to the new one. This is the default | If wrapping fails, the original is not delivered and the sender gets a non-delivery report. Do not use Wrap on rules that affect mail from external senders, as it interferes with Safe Attachments scanning |
| Ignore | The rule is skipped and the message goes out without the text | None needed, but the text is missing |
| Reject | The message returns to the sender as a non-delivery report | Use it for incoming rules instead of Wrap |
The wrapped message also changes what later rules see: rules that read the subject or body will examine the new message, not the original. Microsoft’s advice is to give the disclaimer rule a lower priority than the rules that must see the original, so they run first.
Which option suits an outbound signature depends on how much you mind a missing footer versus a bounced message. The admin walk-through picks Wrap.
The PowerShell version of the Exchange Online mail flow rule signature
Microsoft shows the New-TransportRule cmdlet. Its example adds a disclaimer with an image at the end of every message sent outside the organization, using -SentToScope NotInOrganization and -ApplyHtmlDisclaimerText. Another example puts a month-long banner at the start by adding -ApplyHtmlDisclaimerLocation Prepend with an activation and expiry date. The fallback parameter is -ApplyHtmlDisclaimerFallbackAction.
A rule in that shape, with a fallback, would look like this:
Harbour Studio Ltd. Company number 00000000.New-TransportRule -Name "External footer" -SentToScope NotInOrganization -ApplyHtmlDisclaimerFallbackAction Ignore -ApplyHtmlDisclaimerText "
To find old rules that wrap external mail, Microsoft’s page gives a Get-TransportRule check on ApplyHTMLDisclaimerFallbackAction.
What an Exchange Online mail flow rule signature cannot do
The admin page lists four limits, and the overview adds more.
- It cannot insert the text directly under the latest reply or forward. It goes at the top or bottom of the whole message.
- Users cannot see the added text in their Sent Items folders.
- It cannot embed images.
- Lines with variables it cannot fill are skipped.
- S/MIME encrypted messages cannot have their content changed. For other encrypted messages, transport decryption has to be enabled before a rule can modify the contents.
- System-generated messages, such as non-delivery reports, are not processed.
- A rule with no conditions or exceptions applies to all messages.
- History and changes to rules are not kept, so you cannot revert one to an earlier state. Copy the text somewhere before editing.
- It makes one block for everyone the condition matches. A personal photo, or a different layout per person, is outside what it does.
Microsoft’s tip is that some limits can be overcome with non-Microsoft tools. Our comparison of Microsoft 365 signature management lays that out.
Check that it worked
The Exchange page’s own test list is a good one. Send yourself a plain text email and an HTML email that match the rule, and check the text. Forward them to yourself to confirm the exception stops a second copy. Send some that should not match, and confirm nothing is added. Then try a reply from a phone, since users often write from a different app than the one you tested.
Where an Exchange Online mail flow rule signature fits
An Exchange Online mail flow rule signature is right for text that is identical for everybody: a company registration line, an address or a legal notice. For wording ideas, see email disclaimer examples and the wider email footer guide. If you want the step-by-step version from the company-wide angle, read setting a company email signature in Microsoft 365.
|
Personal details above and a legal line below, the split suggested below. Example only. The company and person are made up.
For the personal part of a signature, a name, a title and a direct number, emailfooter takes a different route: the company designs once, and each employee fills in their own details and follows steps for their mail app. It does not write into Exchange or push to mailboxes, and it is English-only for now. One setup that fits both is an Exchange Online mail flow rule signature for the legal line, with personal signatures above it.
Frequently asked questions
Can an Exchange Online mail flow rule add a signature?
Yes. Microsoft calls it a disclaimer: create a rule with Apply disclaimers and append a disclaimer, or prepend for the top.
How long does an Exchange Online mail flow rule signature take to start working?
Microsoft says a new or modified rule can take up to 30 minutes to be applied to messages.
Why is the Exchange Online mail flow rule signature not under the latest reply?
Microsoft lists that as a limit: you cannot insert the text directly under the latest reply or forward.
Can I use a logo in an Exchange Online mail flow rule signature?
The Exchange page allows an image tag pointing to a hosted file. The admin page says images cannot be embedded.
Can an Exchange Online mail flow rule signature go on outgoing mail only?
Yes. Add a condition such as a recipient outside the organization, because rules apply to incoming and outgoing mail by default.
Keep reading
- How to set a company-wide email signature in Microsoft 365: Set company email signature Microsoft 365 with an Exchange mail flow rule: the admin center steps, tokens, fallback options and the limits you should know.
- Email disclaimer examples: 12 you can adapt, with notes on what each does: Twelve email disclaimer examples grouped by purpose, with length and what each is for, plus how to add one in Google Workspace or Microsoft 365.
- Email footer: what it is and how it differs from an email signature: What is an email footer? A side-by-side table of footer versus signature, a worked example of both on one message, and the admin steps for Google and Exchange.
- Email signature management for Microsoft 365: what the admin center does and does not do: Email signature management for Microsoft 365 per Microsoft’s own pages: what mail flow rules and roaming signatures offer, their limits, and when to add a tool.
Make every signature in your company match
emailfooter is a free WordPress plugin. You design the signature once, and each person adds their own details and installs it in their mail app. Prefer not to run a website? emailfooter Managed is the same software, hosted for you.