In previous posts I have talked about situations where your end users are unable to send or receive email messages under different scenarios. I also posted about the various solutions to those problems. There are a variety of problems and conditions which can create these situations.

One situation where your end users cannot receive email messages but are able to send email involves the configuration of the Exchange server.

Sometimes your end users will come to you and say that they have received the following error message when they try to connect to send email from Outlook or a POP3 (Post Office Protocol) client:

“Messages can be received but not sent. SMTP server unavailable.”

(SMTP stands for Simple Mail Transfer Protocol and is a well known email industry standard. SMTP is used for outgoing mail transport and uses port 25.)

They may also get a variant of that error message such as:

“An unknown error has occurred. Account: ‘SMTP’, Server: ‘Servername’, Protocol: SMTP, Server Response: ‘421 incorrect.domain.com connection limit reached’, Port: 25, Secure(SSL): No, Server Error: 421, Error Number: 0×800CCC67 “

Sometimes you will also see an IP address. The SMTP address, server name and IP address will be different and depend on how your Internet Service Provider (ISP) and email server have been configured.

Another error message you may receive is, “The TCP/IP connection was unexpectedly terminated by your mail server. If this problem persists, contact your LAN Administrator or Internet Provider.”

And this is where you, the email administrator, come in.

This happens because the email server has been inundated with too many connections, more than it can handle. The good news is that if you are having this kind of problem then you know that your network is not the problem. The problem lies with the email server; specifically, you have reached the limit of the maximum number of inbound connections that the email server has been configured to accept.

The flip side to this observation is that you have too many SMTP clients trying to access the email server simultaneously. I’m going to present some steps to overcoming the problem of a maximum inbound connection limit having been reached on the server. But if after increasing the maximum inbound connection limit your users are still having connection issues then you’ll want to start thinking about adding another email server into the mix. This will help to balance the load on your email server with another system on the network.

Now let’s get back to the issue of having a maximum inbound connection limit set too low.

SMTP runs under the Internet Mail Service in your Exchange Server. It has a set limit for incoming connections and the default setting is for thirty (30) connections. Most administrators will kick this up a notch at install time. If you’re lucky you will already have an idea of what this setting should be.

This topic is an opening into the discussion of how to size your email servers based on the number of users you expect will be making connections to your server, the average size of attachments they will be sending and receiving, and the peak hours, days, etc of average email service utilization . But like I said that is a separate discussion.

If you want to increase your maximum inbound connection value on the Exchange server then you will need to bring up the Exchange Server Administrator program. On the left hand side locate your organization name and site location. Next, click on “Connections”. On the right hand side you will want to double-click on “Internet Mail Service” to bring up the properties. Click on the “Connections” tab and then follow that by clicking on “Advanced”. Here is where you will be able to modify the Maximum Number of Inbound Connections setting. Increase it by adding 50 or 100 or some other number to the current value. You will have to save the settings and then have your users try reading their email to see if you have added enough connections to support their usage. You might have to loop on this procedure a couple of times to set the right amount on inbound connections for your organization.

There are tools on the market which can simulate a load against a server such as an email server. You might want to investigate those tools first before asking your email end users to participate in your exercise for finding the best inbound connection threshold for your organization.