Open Exchange Management Shell and start by checking your Send Connector(s) :
Get-SendConnector
You should get the name of your send connector(s) – like “Default Send Connector”.
If you haven’t messed up with the port configuration of the connector yet,
it should be using the default port -25. It is a good idea to confirm this with the command:
Get-SendConnector | fl port

Then change the port with the command:
Set-SendConnector –Identity “Your Send Connector Name” -Port 587

If you have only one Send Connector you can use that command too:
Get-SendConnector | Set-SendConnector -Port 587

Of course, it is a good idea to check the final result again with:
Get-SendConnector | fl port

Or even take a look at the full listing for the send connector:
Get-SendConnector | fl