Relay Application Server SMTP Traffic is getting time out

I was dealing with a Email Relay application which was sending 5000-10000 news letters in one shot to internal recipients & External.

Application team complain that the distribution often fail with below error and need to check from Exchange side. This kind of behavior have been seen specially with the Environment of Exchange 2001 because of shadow redundancy feature, While it works well with Exchange 2007 servers.

Error which received in application :


System.Net.Mail.SmtpException: The operation has timed System.Net.Mail.SmtpClient.Send(MailMessage message)


Resolution :

The delayed acknowledgement time-out is controlled by the MaxAcknowledgementDelay attribute of each Receive connector. The default value is 30 seconds.

In my case we have created a custom receive connector called “Email Relay” which is accepting relay emails from application. application IP address is already added in source of receive connector. I will not talk too much about how we configure custome receive connector in exchange 2010.

The receive connector attributes changes with new value including MaxAcknowledgementDelay attribute to resolve this.

Old Settings :

MaxInboundConnection                    : 5000

MaxInboundConnectionPerSource           : 20

MaxInboundConnectionPercentagePerSource : 2

TarpitInterval                          : 00:00:05

MaxAcknowledgementDelay                 : 00:00:30

MessageRateLimit                        : unlimited

MessageRateSource                       : IPAddress


New Settings :

MaxInboundConnection                    : 5000

MaxInboundConnectionPerSource           : 100

MaxInboundConnectionPercentagePerSource : 20

TarpitInterval                          : 00:00:00

MaxAcknowledgementDelay                 : 00:00:00

MessageRateLimit                        : unlimited

MessageRateSource                       : None

Use below cmdlet to change it:

Set-ReceiveConnector “HUB server\EMail Relay” -MaxAcknowledgementDelay 0 -TarpitInterval 0 -MessageRateSource None


Note – Although disabling delayed acknowledgements for a Receive connector increases SMTP throughput, it also means that you no longer benefit from the features provided by shadow redundancy. For this reason, Microsoft recommend the use of storage hardware redundancy for transport servers for which delayed acknowledgements are disabled.  So its depend upon your IT management decision for such changes.

A good approach is to first lower the value and then verify whether SMTP throughput still suffers and, if it does, then disable the feature completely.

Hope above information will help you 🙂

Read some more information:

Mike :

TechNet :





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