External Warning for Office 365 External Emails

This script will prepend 'External' in subject and add 'Disclaimer' for external emails.

External Warning Rule for External Emails/ExternalWarningRule.ps1

@@ -0,0 +1,112 @@
+#Create mail flow rule for adding External warning message for external mails
+Param
+(
+    [Parameter(Mandatory = $false)]
+    [switch]$MFA,
+    [string]$ExcludeGroupMembers,
+    [string]$ExcludeMB,
+    [string]$UserName,
+    [string]$Password
+)
+Get-PSSession | Remove-PSSession
+#Authentication using MFA
+if($MFA.IsPresent)
+{
+ $MFAExchangeModule = ((Get-ChildItem -Path $($env:LOCALAPPDATA+"\Apps\2.0\") -Filter CreateExoPSSession.ps1 -Recurse ).FullName | Select-Object -Last 1)
+ If ($MFAExchangeModule -eq $null)
+ {
+  Write-Host  `nPlease install Exchange Online MFA Module.  -ForegroundColor yellow
+  Write-Host You can manually install module using below blog : `nhttps://o365reports.com/2019/04/17/connect-exchange-online-using-mfa/ `nOR you can install module directly by entering "Y"`n
+  $Confirm= Read-Host `nAre you sure you want to install module directly? [Y] Yes [N] No
+  if($Confirm -match "[Y]")
+  {
+   Start-Process "iexplore.exe" "https://cmdletpswmodule.blob.core.windows.net/exopsmodule/Microsoft.Online.CSE.PSModule.Client.application"
+  }
+  else
+  {
+   Start-Process 'https://o365reports.com/2019/04/17/connect-exchange-online-using-mfa/'
+   Exit
+  }
+  $Confirmation= Read-Host Have you installed Exchange Online MFA Module? [Y] Yes [N] No
+  if($Confirmation -match "[yY]")
+  {
+   $MFAExchangeModule = ((Get-ChildItem -Path $($env:LOCALAPPDATA+"\Apps\2.0\") -Filter CreateExoPSSession.ps1 -Recurse ).FullName | Select-Object -Last 1)
+   If ($MFAExchangeModule -eq $null)
+   {
+    Write-Host Exchange Online MFA module is not available -ForegroundColor red
+    Exit
+   }
+  }
+  else
+  {
+   Write-Host Exchange Online PowerShell Module is required
+   Start-Process 'https://o365reports.com/2019/04/17/connect-exchange-online-using-mfa/'
+   Exit
+  }
+ }
+
+ #Importing Exchange MFA Module
+ . "$MFAExchangeModule"
+ Write-Host Enter credential in prompt to connect to Exchange Online
+ Connect-EXOPSSession -WarningAction SilentlyContinue
+}
+
+#Authentication using non-MFA
+else
+{
+ #Storing credential in script for scheduling purpose/ Passing credential as parameter
+ if(($UserName -ne "") -and ($Password -ne ""))
+ {
+  $SecuredPassword = ConvertTo-SecureString -AsPlainText $Password -Force
+  $Credential  = New-Object System.Management.Automation.PSCredential $UserName,$SecuredPassword
+ }
+ else
+ {
+  $Credential=Get-Credential -Credential $null
+ }
+ $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $Credential -Authentication Basic -AllowRedirection -WarningAction SilentlyContinue
+ Import-PSSession $Session -AllowClobber -DisableNameChecking | Out-Null
+}
+$Disclaimer='<p><div style="background-color:#FFEB9C; width:100%; border-style: solid; border-color:#9C6500; border-width:1pt; padding:2pt; font-size:10pt; line-height:12pt; font-family:Calibri; color:Black; text-align: left;"><span style="color:#9C6500"; font-weight:bold;>CAUTION:</span> This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.</div><br></p>'
+$ExcludeGroups=@()
+$ExcludeMBs=@()
+
+#Check for exclude groups
+if([string]$ExcludeGroupMembers -ne "")  
+{  
+ $Groups= $ExcludeGroupMembers -Split ","
+ #Check whether the grup exists
+ foreach($Group in $Groups)
+ {
+  $check=Get-DistributionGroup -Identity $Group -ErrorAction SilentlyContinue
+  if($check -eq $null)
+  {
+   $check=Get-UnifiedGroup -Identity $Group -ErrorAction silentlycontinue
+   if($check -eq $null)
+   {
+    Write-Host $Group not exist in the tenant -ForegroundColor Red
+    continue
+   }
+  }
+  $ExcludeGroups +=$Group
+ }
+ Write-Host `nCreating mail flow rule for External Senders... -ForegroundColor Yellow
+ New-TransportRule "External Email Warning" -FromScope NotInOrganization -SentToScope InOrganization -PrependSubject [EXTERNAL]: -Priority 0 -ApplyHtmlDisclaimerText $Disclaimer -ExceptIfSentToMemberOf $ExcludeGroups -ApplyHtmlDisclaimerLocation Prepend -ApplyHtmlDisclaimerFallbackAction Wrap
+}
+#Create Transport rule with exclude mailbox that in To and CC
+elseif($ExcludeMB -ne "")
+{
+ $MBs= $ExcludeMB -Split ","
+ #Check whether the grup exists
+ foreach($MB in $MBs)
+ {
+  $ExcludeMBs +=$MB
+ }
+ Write-Host `nCreating mail flow rule for External Senders... -ForegroundColor Yellow
+ New-TransportRule "External Email Warning" -FromScope NotInOrganization -SentToScope InOrganization -PrependSubject [EXTERNAL]: -Priority 0 -ApplyHtmlDisclaimerText $Disclaimer -ExceptIfAnyOfToCCHeader $ExcludeMBs -ApplyHtmlDisclaimerLocation Prepend -ApplyHtmlDisclaimerFallbackAction Wrap
+}
+else
+{
+Write-Host Creating mail flow rule for External Senders... -ForegroundColor Yellow
+New-TransportRule "External Email Warning" -FromScope NotInOrganization -SentToScope InOrganization -PrependSubject [EXTERNAL]: -Priority 0 -ApplyHtmlDisclaimerText $Disclaimer -ApplyHtmlDisclaimerLocation Prepend -ApplyHtmlDisclaimerFallbackAction Wrap
+}