telnet:25 on the right DNS entry ???

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example: myemail at usa.net
I tried randomly until I found that to use telnet on the port 25(smtp) the
right address is NOT usa.net but it is mail.usa.net instead...
how should a software understand it????

again: it I do telnet usa.net 25 IT WON'T WORK !!!
I manually guessed mail.usa.net 25 and it's fine but the PC does now
guess...
what C/C++ function returns or redirects to the right DNS entry ???