Purview DLP Behaviours in Outlook Desktop

Hi,

These are known issues 

Override without justification (Send Anyway without selecting a reason)

This is actually expected behavior unless you explicitly enforce justification via configuration. By default, Outlook presents the oversharing dialog but does not gate the send action on the user completing a justification. To enforce this you need to configure the following.

GPO:

Classic Outlook
Specify wait time to evaluate sensitive content at Software\Policies\Microsoft\office\16.0\Outlook\options\Mail
Registry:
Key: DLPWaitOnSendTimeout (DWORD)

New Outlook
Registry via Exchange Online Mailbox parameter in Powershell (there is no Policy option)

Connect-ExchangeOnline 
#for the entire organization 
Set-OrganizationConfig -DLPWaitOnSendEnabled $true -DLPWaitOnSendTimeout 25 
#for specific mailbox only 
Set-Mailbox -Identity "email address removed for privacy reasons" -DLPWaitOnSendEnabled $true -DLPWaitOnSendTimeout 25 
#verify the setting 
Get-OrganizationConfig | Select-Object DLPWaitOnSendEnabled, DLPWaitOnSendTimeout

On the timeout value the following:

0 = user immediately sees "Send Anyway" — effectively no enforcement
1–9998 = Outlook waits that many seconds for evaluation to complete before allowing override (25–60 seconds is a practical range)
9999 (default) = mail will not be sent until DLP evaluation fully completes — strictest option

Inconsistent policy tip display

The two build you mentioned (Click-to-run vs MSO) are different build types and can render the oversharing dialog differently. This is a known inconsistency in classic outlook across mixed build environments.

On a broader note

Classic Outlook is on a deprecation path, so rendering inconsistencies are unlikely to receive dedicated fixes. New Outlook has a more consistent and actively developed experience; migration is worth considering as a longer-term solution.