By default Exchange 2016 will attempt to connect to the AuthSMTP servers on the default SMTP port 25, if you wish to change this to one of the Alternative ports (2525, 23, 26) you will need to take the following steps.
Open the 'Exchange Management Shell'
Type the following command to get a list of all your send connectors:
To change the SMTP port for your connector you will need to enter the following command replacing 'AuthSMTP' with the appropriate connector name from the above command
Set-SendConnector -Identity "AuthSMTP" -Port 2525
To confirm the change has been accepted you can enter the following command and it will return the current configured port, this should match what you have set
Get-SendConnector "AuthSMTP"|fl Port
For the changes to take effect you will need to restart the SMTP / transport services on your Exchange server.
You should now be able to send messages via AuthSMTP on the alternative port 2525
If you have questions specifically about configuring or troubleshooting Exchange 2016 please use the Official Microsoft Exchange 2016 Documentation - due to the complexities of Microsoft Exchange we can only provide basic troubleshooting advice.