how does sap system receive e-mail , the port:25 can not used to run smtp

how does sap system receive e-mail? It seems that sap system can not use port:25 to run smtp server.

I add a parameter in profile:

icm/server_port_1 PROT=SMTP,PORT=25001,TIMEOUT=180

then I send a mail to sap system , but all the mails only know the port:25 , how can my e-mail know that sap system uses port:25001 to receive e-mail.

thanks