I'm using postfix to send emails. So, and correct me if I'm wrong, I'm using port 25 TCP inbound for entering emails, and port 25 TCP outbound for sending emails. Is it possible to alter the 25 outbound port value used by postfix to 60000 for example?
You are listening on port 25 for incoming emails.
You are connecting to a remote server on port 25 for outgoing emails.
The port you connect from is randomly generated.
You have to modify the transport table using the command postmap. With postmap you can modify the way postfix interacts with the other servers for the outgoing message.
See http://www.postfix.org/transport.5.html in order to understand how you can accomplish this for a particular user/recipient/domain as almost every public SMTP server is listening to 25 and 587 TCP only!