title: Find out which mail server is installed on Ubuntu
body: <p>I have access to a couple of Ubuntu servers and I am trying to configure the PHP mail() command for each of these servers. However, for one of these servers, the From: header is overwritten by www-data@. I have no access to the SPF record of that hostname, so I want to configure the mail server so it doesn't overwrite the From: header.</p>
  
  <p>So my question is, how do I find out which mail server is used so I can correctly configure that mail server? (Postfix, SSMTP, ...)</p>
  
  <p>Thanks in advance!</p>