If you are getting a message in the logs something like:
Socket error: 13 – Permission denied getemails: There was an error connecting to the server Permission denied
And you are running SELINUX then likely you need to give Apache permission to connect
You do this by changing the httpd_can_network_connect variable. The httpd_can_network_connect variable has this documentation: Allow HTTPD scripts and modules to connect to the network
Run the following command while logged into the same server that has Postie on it:
setsebool -P httpd_can_network_connect 1