To try and attempt to reduce phising attempts it is possible to attend a HTML warning to users when they receive emails from outside the organisation. This is overkill in many environments but some insist on it being in place.
It's achieved using Transport Rules in Exchange Online, the code below implements a warning
"Caution: 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."
//Create a Session to Exchange Online
Write-Host "Getting the Exchange Online cmdlets" -ForegroundColor Yellow
$Session = New-PSSession -ConnectionUri https://outlook.office365.com/powershell-liveid/ `
-ConfigurationName Microsoft.Exchange -Credential $credentials `
-Authentication Basic -AllowRedirection
Import-PSSession $Session -AllowClobber
//Create a new Transport Rule
New-TransportRule -Name "External Mail Warning - Outside Organisation" -Priority 0 -FromScope "NotInOrganization" -ApplyHtmlDisclaimerFallbackAction Ignore -ApplyHtmlDisclaimerLocation "Prepend" -ApplyHtmlDisclaimerText "<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>"