How to diagnose inbound message delivery delays


Determining the cause of inbound message delivery delays.


Messages arrive to MailEnable via the SMTP Connector. To determine the time that the message was received by the SMTP Connector requires inspection of the SMTP logs. If the message arrival is not indicated in the log files, the problem is either an issue with public DNS configuration, or a firewall is blocking connectivity. Try for confirmation.

Then, inspect the MTA log to determine at what time the message was moved to the post office connector.

Finally, inspect the post office connector log to determine the time that the message was delivered to the post office/mailbox. This will confirm whether the messages are taking time to be delivered to the mailbox or if the POP (or other message access protocol) service is not identifying that the message has been delivered to the Mailbox (this is unlikely to be the case, since it would be quite a significant issue).

Further instruction on tracking messages via the log files are outlined below:

Also, be aware that with most mail access protocols, the mail client is responsible for polling the mailbox to determine if new messages have arrived. There is no way for the server to notify the client as such. The logs will show how often the client is polling the server for new messages. Another good test is to mail a message to the mailbox via web mail and open the mail client (which will poll the mailbox immediately).

There are a large number of configuration options which can affect the speed of receiving and delivery of messages, whether they be to local or remote addresses. This document outlines some of these items.

For SMTP inbound:

Option Effect Resolution
Authenticated senders must use valid sender address When authenticating, MailEnable has to check if the senders address is a valid email for the post office, which requires a scan of all the email addresses on the server Only use this if needed. If mail users are unknown this can prevent valid users from spamming, since they have to send from their valid address.
Reverse DNS blacklisting a DNS lookup has to be performed for each inbound connection. It is not recommended to enable more than a couple of blacklists if the server receives a large amount of mail
Large number of email addresses/domains/postoffices MailEnable has to scan through each of the items find a match Performance is hardware dependent, and also whether users have permission to alter their configuration via a control panel. If running over 50,000 addresses, performance can be greatly enhanced by using the Enterprise version of MailEnable which stores configuration information in a database.
IP addresses in the Access Control MailEnable has to scan through all the IP entries in the list for each incoming connection Large performance improvements in version 1.19 of Pro and 1.8 of Standard. It is not advisable to have a large number of items though. If the auto-ban IP addresses is enabled in the SMTP options, check the blocked IP addresses regularly.
Require PTR DNS entry for unauthenticated connections MailEnable has to perform a DNS lookup on the connecting IP address Not a large performance impact.
Reject mail if sender address is from invalid domain MailEnable has to perform a DNS lookup on the domain of the sender Not a large performance impact.

For SMTP outbound:

Option Effect Resolution
Large amount of delayed items in the outbound queue MailEnable has to read the command file of each message when checking to see what items to send. Delayed items increase the time and disk I/O. Thousands of items in the outbound queue that have been delayed will severely impact the delivery of emails. Make sure that the items in the SMTP outbound are there for a valid reason. If you are blocked by AOL for instance, you may have a lot of items which have been delayed.
Sending multiple messages to the same domain which is tarpitting you Some remote mail servers will slow down the connection if they detect a large volume of email originating from an IP address. This can tie up all outbound SMTP threads if several messages are still being sent to the remote server. Check to see if the messages are spam, and delete them if so. The MEQueueClean application available from our download pages can assist with this.
Poor DNS Performance MailEnable performs DNS lookups whenever sending items to remote mail servers. The longer it takes to perform a lookup, the slower the delivery. There should be minimal time delays when performing DNS lookups

If the mail server is running slow, there is usually two main reasons. Hard disk I/O and queue size. Hard disk I/O can be measured with Perfmon.


How to control how often users are sent message delay notifications:

How to diagnose outgoing mail problems and Delivery Delay Notifications:

Error "Message Delivery has been delayed":

Product:MailEnable (All Versions)
Class:TRB: Troubleshooting (Configuration or Environment)
Revised:Wednesday, May 4, 2016