I’ve seen several cases where customer Exchange systems suddenly stop receiving mail. The most common causes are relatively simple to resolve but are often missed by a panicked IT admin, extending the period of end user impact.
Potential Issue #1: You get reports of bounce messages containing “452 4.3.1 Insufficient system resources”
Cause: The Volume containing the Exchange Transport Queue may be out of disk space link to fix 452 4.3.1 Insufficient system resources.
Potential Issue #2: The volume containing the transaction logs is close to full. Exchange will limit mailflow if the log volume gets near full (within a couple gigs).
Cause: backups are not running successfully and clearing the logs
Quick Resolution: If your Exchange Server is virtualized or you are using a SAN, then you can simply enlarge the volume that stores the Transaction Logs.
Alternative Resolution: Enable circular logging to clear the transaction logs (requires restarting information store service).
Circular logging is a temporary measure that makes Exchange automatically deletes the transaction logs rather than waiting for successful backup to complete. The Circular logging checkbox can be found under the properties of the Exchange database. Note: I recommend you do not manually delete transaction logs unless there are no other options available.
Long term resolution: fix backups – make sure you have an Exchange aware backup solution that can quisque the Exchange databases and back them up.
Potential Issue #3: Some of the Exchange services are stopped
Resolution: Open services.msc and scroll down to the ‘Microsoft Exchange…’ section and make sure any services with a startup type of ‘Automatic’ are set to status ‘Started’. In the case of Exchange 2003, make sure the Simple Mail Transfer Protocol service is running. If you are using a spam filtering solution installed locally on the Exchange server, make sure it’s services are also started.
Potential Issue #4: The Exchange databases are dismounted:
Resolution: Open Exchange Management Console and review the database status and make sure all databases are mounted.
Potential Issue #5: DNS, Firewall, or Spam Filtering related issues.
This is outside the scope of this article, but I will provide some general direction.
In terms of inbound mail, I suggest you review your internet DNS records (MX Record) to see what IP address it points to, confirm that is your firewall or third party spam filtering solution and review and test the inbound mail path.
Logon to the management of your spam filtering solution and confirm it can send messages to your Exchange server and is receiving messages from the outside world.
If you have Exchange 2007,2010 or 2013 then review your Receive connectors to confirm they allow inbound mail from your spam filtering solution, or a quick fix is to simply check ‘anonymous’ on the Receive connector settings. However look up this setting to confirm you understand its use. Receive connectors can be found under Server Configuration > Hub Transport.
In terms of outbound mail, I would confirm the mail server can get out over port 25 by doing a simple ‘telnet’ test. “telnet knowngoodoutsidemailservername.com 25″
if this works you should see something in your command prompt when a connection is made.
Review your Send Connector in Exchange and check the ‘smart host’ setting to see if Exchange is relaying your outbound mail someplace before its send out to the internet.