We recently migrated email service providers and as part of the service cutover I needed to check mailflow was working correclty for all hub transport servers in our DAG. In order to do this with minimal disruption and without ‘failing over’ manually to each transport server to verify all was working as expected, I used good ol telnet instead to assist me with the testing as follows:

Ensure the pc you will be connecting from is added as a receive connector on each hub transport server, in order to do so, you’ll need to issue the following powershell command with the following permissions:

New-ReceiveConnector -Name "Test" -Usage Custom -AuthMechanism Tls, ExternalAuthoritative -PermissionGroups AnonymousUsers, ExchangeServers -Bindings -RemoteIpRanges

Once the receive connector has been created you should then be able to log onto the exchange server with the following telnet command:

telnet TransportServer.domain.local 25

Hopefully you should now receive a ‘service ready’ message after the initial telnet login command. Now that we’re connected to the server we can start issuing commands as follows:


This should return some information to let us know we’re connected to an email server with certain capabilities. Finally we can test send an email from the telnet command line as follows:

MAIL FROM: internal@internaldomain.com
RCPT TO: external@externaldomain.com
message content

Once your message has been successfully sent it should then just be a simple case of checking the message headers once your message has arrived at its destination to confirm the path of the mail flow is as expected.

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