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Symptoms
In Microsoft Exchange Server 2019, 2016, or 2013, email messages may be stuck in on-premises message queues for several minutes if the server is configured to send to a single destination, such as Exchange Online. There are few or no deferrals (400-series SMTP response codes) from Exchange Online to account for the number of messages in the queue. Eventually, the messages are sent. However, there are some delays.
Cause
Exchange Server is designed to create concurrent or parallel connections to send messages to multiple destinations. To make sure that those connections are not exhausted by sending to a single destination at the expense of others, the Exchange Server default settings restrict sending lots of mail to a single destination (FQDN of SendConnector/NextHopDomain), such as Exchange Online. If a server that is running Exchange Server is configured to send all messages to Exchange Online for relaying or as part of hybrid configuration, these settings limit the number of parallel and total connections that are created. This, in turn, limits the throughput of messages to Exchange Online.
Resolution
If your Exchange-based servers are primarily used to send to Exchange Online, you can change the following settings to optimize performance and avoid building large queues.
SmtpConnectorQueueMessageCountThresholdForConcurrentConnections
The SmtpConnectorQueueMessageCountThresholdForConcurrentConnections parameter determines how many messages in a queue will trigger the creation of another connection to the destination. The lower the threshold, the sooner that Exchange Server opens a new connection to Exchange Online. For a large volume of email, this translates to fewer messages in the queue because there will be more connections in parallel to transmit messages from the queue. The default value is 20 messages. You can set this value to 2 for the highest throughput. To do this, open the Edgetransport.exe.config file, and add the following parameters anywhere after the <AppSettings> tag on all servers that handle traffic to Exchange Online:
<add key="SmtpConnectorQueueMessageCountThresholdForConcurrentConnections" value="2"/>
Note
You must restart the MSExchangeTransport service for changes to take effect.
MaxPerDomainOutboundConnections
The MaxPerDomainOutboundConnections parameter specifies the maximum number of concurrent connections to any single domain. The default value is 20 connections. To increase the maximum number of connections, run the following cmdlet:
Set-TransportService Mailbox01 -MaxPerDomainOutboundConnections 40
MessageRetryInterval
The MessageRetryInterval parameter specifies the retry interval for individual messages after a connection failure to a remote server occurs. The default value is 15 minutes. To reduce the retry interval value, run the following cmdlet:
Set-TransportService Mailbox01 -MessageRetryInterval 00:05:00
For more information, see Set-TransportService.