Enabling the External Recipient MailTip in Exchange Online

The External Recipient MailTip is not enabled by default in Exchange Online, but it’s useful for warning users when they are about to send an email to someone outside your organisation.

Benefits

The External Recipient MailTip offers several benefits:

Prevents Unintended Information Disclosure — It alerts users when they are about to send an email to recipients outside their organisation, helping prevent accidental sharing of sensitive or confidential information.

Enhances Security Awareness — By notifying users about external recipients, it raises awareness of potential phishing or impersonation attempts where someone might be pretending to be an internal contact.

Improves Communication Quality — Knowing that an email will be sent outside the organisation encourages users to review the content, tone, and wording of their messages to ensure they are appropriate for external audiences.

Consistency Across Clients — The MailTip feature works across Outlook desktop, Outlook on the web (OWA), and Outlook mobile, giving you consistent coverage without third-party plugins.

What It Looks Like

The external recipient mailtip appears as a yellow banner at the top of the message when composing an email to an external address:

Outlook (classic) — A yellow bar reads “This recipient is outside your organisation.”

Outlook (new) / OWA — A similar yellow banner displays the same warning above the recipients list.

Outlook (mobile) — The mailtip is shown inline in the compose view.

Default Configuration

You can check your current MailTip settings in Exchange Online PowerShell:

Get-OrganizationConfig | fl Mailtips*

The default output looks like this:

MailTipsAllTipsEnabled               : True
MailTipsExternalRecipientsTipsEnabled : False
MailTipsGroupMetricsEnabled           : True
MailTipsLargeAudienceThreshold        : 25
MailTipsMailboxSourcedTipsEnabled     : True

These settings control the following:

Setting	Description	Default
`MailTipsAllTipsEnabled`	Master switch for all MailTips	True
`MailTipsExternalRecipientsTipsEnabled`	Warns when composing to external recipients	False
`MailTipsGroupMetricsEnabled`	Enables MailTips that rely on group metrics data	True
`MailTipsLargeAudienceThreshold`	Number of recipients that triggers a “large audience” warning	25
`MailTipsMailboxSourcedTipsEnabled`	Enables MailTips based on mailbox data (out-of-office, full mailbox)	True

Enabling the External Recipient MailTip

MailTips are enabled by default as a feature, but the external recipients tip is disabled by default. To enable it, run:

Set-OrganizationConfig -MailTipsExternalRecipientsTipsEnabled:$True

This applies to all users across all Outlook clients — desktop, OWA, and mobile — immediately (allow a few minutes for propagation).

Large Audience Threshold

The default threshold of 25 recipients may be too low for organisations that regularly send to distribution groups. Adjust it with:

Set-OrganizationConfig -MailTipsLargeAudienceThreshold 50

This will show a warning when a user sends to more than 50 recipients, reminding them to use Bcc or a distribution list.

Disable Automatic Replies MailTip

If you don’t want users to see out-of-office MailTips (e.g. to reduce noise for heavily automated mailboxes):

Set-OrganizationConfig -MailTipsMailboxSourcedTipsEnabled:$False

Troubleshooting

MailTip not showing for external recipients

Verify the setting took effect: run Get-OrganizationConfig | fl MailTipsExternalRecipientsTipsEnabled — it should return True.
Wait up to 30 minutes for replication across Exchange Online.
Check that the recipient’s domain is external to your tenant — mailtips won’t show for internal recipients.

MailTip still showing after disabling

Run Set-OrganizationConfig -MailTipsExternalRecipientsTipsEnabled:$False to confirm the change.
Changes are typically fast, but in some cases can take up to an hour to propagate.

Users complain about the warning

You cannot customise the text of the external recipient mailtip — it’s hardcoded by Microsoft. If you need a custom message, consider adding a transport rule to insert a disclaimer instead.