setting SMTP server privileges

Hello

I am not sure if this is the correct place for this question, and if not, please move this to the correct forum

I have Linux machine that is capable of sending emails via sendmail
I have another Windows machine on which I have PHP installed
I would like PHP on the Windows machine to use the SMTP server on the Linux machine for sending emails
I configured the IP address of the Linux machine as the SMTP server but I still cannot get it to work

I am thinking that there is some privileges configuration on the Linux that only allows it to accept SMTP requests from itself.
If that is the case, does anyone know where such configuration be and how can I configure my Linux machine to also accept SMTP requests from my Windows Machine?

regards and thanks in advance