Exchange 2010/2013 Event ID 1021 Warning "Receive connector rejected an incoming connection from IP address"

We started getting a few 1021 Warnings on our Exchange 2013 servers after ramping up our Anonymous Relay traffic. This means that you have a large number of connections from the same IP.

The default maximum concurrent connections from the same IP for Receive Connectors is 20...we needed that to be higher so it wouldn't reject messages!

**Note** Make sure you recognize the IP that being presented in the warning; it could be a spammer, and you wouldn't want to make the following changes unless you're sure that the volume of connections is normal.

The full warning is:

Log Name:        Application
Source:              MSExchangeFrontEndTransport
Date:                 3/17/2016 11:25:15 AM
Event ID:          1021
Task Category: SmtpReceive
Level:               Warning
Keywords:        Classic
User:                 N/A
Computer:         MBX1.exchangeitup.com
Description:
Receive connector Internal Anonymous Relay rejected an incoming connection from IP address x.x.x.x. The maximum number of connections per source (20) for this connector has been reached by this source IP address.

To check the current limits on Receive Connectors, fire up the EMS and run:

Get-ReceiveConnector | select name, *MaxInbound*

You'll get something like this:

As you can see, my "Internal Anonymous Relay" connectors are set to 20 for MaxInboundConnectionPerSource, and 2 for MaxInboundConnectionPercentagePerSource.

We want to bump those up for the "Internal Anonymous Relay" on all servers, so we'll run the following cmdlet:

Get-ReceiveConnector *\"Internal Anonymous Relay" | Set-ReceiveConnector -MaxInboundConnectionPerSource 500 -MaxInboundConnectionPercentagePerSource 100

**Note** You'll need to change "Internal Anonymous Relay" to your Receive Connector name.

This will set maximum number of inbound connections from a single source IP to 500 and maximum amount (in percentage) of connections remaining that can get used by a single source IP to 100.

**Note** You can set the max number of connections to a higher value or unlimited but you'll want to go in small increments to test for adverse effects.

Now run the Get-ReceiveConnector | select name, *MaxInbound* cmdlet again, and you'll get:

Now you'll stop getting warnings and connections won't be rejected.