This blog post focusses on an issue where your Exchange Online users are not able to send emails to other Exchange Online recipients outside of your organization when using a 3rd Party Centralized Email Flow Setup. The term 3rd Party Centralized Email Flow Setup describes a solution where you not follow the preferred hybrid architecture proposed by the Exchange product group, but use a 3rd party software as a centralized email gateway.
You have followed the recommendation to secure the Exchange Online inbound connector for your on-premises email servers by using a certificate name or the remote IP address of your on-premises email gateway.
The on-premises email security gateway utilizes a self-signed certificate to secure TLS connections. The gateway is configured to use two different send connector setups:
In this case Exchange Online Protection (EOP) will not be able to differentiate between tenant internal inbound mail flow and mail flow targeted to other tenants. Therefore, email messages sent from your Exchange Online users to recipients located in other Exchange Online tenants will be discarded.
Interestingly enough, this will happen silently. Your gateway solution will log a successful delivery to Exchange Online Protection. The tenant administrator of the recipient domain will not find an any information in the Exchange Online message tracking logs.
The following diagram illustrates the setup.
The solution for this problem is pretty simple. Just use dedicated certificates for each connector targetting Exchange Online.
Change the Internet Connector to fully trusted 3rd party certificate. In this case you are not required to modify the existing Exchange Online inbound connector setup.
The new connector configurations are:
The following diagram illustrates the new setup:
Out-of-Office (OOF) messages have to follow the compliance rules as regular email communication. This is not necessarily a known fact to end users.
If a company does have a strict compliance policy regarding external OOF messages you can use the following solution to establish a strict and simple to use OOF configuration.
Only specific employees are supposed to send OOF messages to external recipients. All other employees are supposed to send internal OOF messages only.
The solution consists of two PowerShell scripts.
The first script is used to remove any exisiting OOF rules created by a user using the Outlook OOF Rule Wizard. This is required to avoid any strange behaviours in regards to OOF messages being sent even if OOF is deactivated. The most common reasons for such a behaviour are migrated OOF rules created by previous Exchange Server versions.
You can read more about scripts here:
You can use the follow command line example, if you want to automate the execution of script 2 using a scheduled task.
C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe -version 3.0 -command ". 'D:\Program Files\Microsoft\Exchange Server\V15\bin\RemoteExchange.ps1'; Connect-ExchangeServer -auto; "D:\Scripts\Set-ExternalOOF\Set-ExternalOOF.ps1 -RemoveRights"
This script searches for OOF rules created by users using the Outlook rule-tab in the OOF assistant and deletes exisiting OOF rules.
In preparation to configure compliant Out-Of-Office (OFF) settings for users, any existing OOF rule needs to be deleted. The script will use either an exisiting Exchange Server EWS library or the Managed EWS library installed using the default file path.
This is the first of two scripts for the complete solution. Find the second script here.
The script access the mailbox rules using Exchange Web Services. Therefore the account executing the script either needs to have ApplicationImpersonation rights or full access to the user mailbox.
# EXAMPLE 1
# Find any existing OOF rule and write results to log file
# EXAMPLE 2
# Find and delete any existing OOF rules in all user mailboxes and write delete actions to log file
# EXAMPLE 3
# Find and delete any existing OOF rules for user SomeUser@varunagroup.de and write delete actions to log file
Remove-OOFRule -Mailbox SomeUser@varunagroup.de -Delete
Rhoderick Milne (https://blogs.technet.microsoft.com/rmilne)
This script sets the mailbox ExternalOofOptions to 'External' for members of a given security group.
ExternalOofOptions for users that are NOT a member of the security group will be set to 'InternalOnly'. If required the script will set the ExternalAudience to None and will delete an existing OOF message.
Controlling the ExternalOofOptions and ExternalAudience settings has been implemented to follow dedicated company compliance rules.
This is the second of two scripts for the complete solution. Find the first script here.
# Run script with default settings