If you're using mail functions on PHP after just installing it, you won't get any error message, even though the mail won't get sent yet. Don't worry, we'll fix that now.

Debian comes with Exim4 which is a mail server with a bunch of features. By default it's set to only send emails locally (to other users on your system). If you tried to send any emails to external users at the moment, you'll see something like "** www-data@debian R=nonlocal: Mailing to remote domains not supported" in your error logs.

We just need to reconfigure your Exim4 options, which you can do by typing the command "dpkg-reconfigure exim4-config".  Then you'll get a screen, which you'll have to answer a few options. A lot of them are easy but they can be a bit off putting.

I don't think there's much reason to split configuration files (comment if I'm wrong). Your system mail name should be your URL, including the subdomain if that's where you've pointed your VPS. For instance, mine is "trinity.allroundnews.co.uk", so all email addresses will be 'user@trinity.allroundnews.co.uk'. Let the 'listener daemon' listen to all IPs, by just leaving it blank. Say no to 'Dial-on-Demand'. The 'Root and postmaster mail recipient' is basically the user that you want mail to 'root@trinity.allroundnews.co.uk' to be redirected to (since you shouldn't stay logged in as root). So for me, that's just 'shamess'.

If you've tried to send any mail so far, you'll want to remove it from the queue (sorry if it was important). Do that by getting up the mail queue (using the mailq command). Then you can remove mail by using the command Exim -Mrm <message id>. You can put as many message ID's in the list as you like, space separated. The message ID is the weird string that looks something like ’1JOJBI-0000ag-IH'. Yup, you have to type that.

I'll talk about actually reading mail later, but at least now you can send mail with PHP!