I do have quite a few customers who still have PHP websites that use the quite old mail function.
This function uses the local sendmail application on UNIX based systems to forward mails.

Therefore if the responsible mailserver for a given domain is not installed on the same server the mail will be rejected by most recipients. Two possible solutions exist:

1. You can add the webserver as an additional authorized mail server…
2.  … or configure sendmail to relay mails through your already working mailserver.

I always choose option 2, as it’s easier to configure and maintain. You only have to have a valid SMTP user for the existing mailserver and root access to the webserver. The following steps show how to set mail relaying up:

Install sendmail

Before we can start we have ensure, that sendmail is installed. This can be running the following command:

whereis sendmail

If it is installed the full path to the executable will be printed. Otherwise you can install it using your distributions’s package manager. On Debian-style distributions you can use the following command:

sudo apt-get install sendmail

The installation can take quite some time on slow internet connections.

Configure relay credentials

Now we have to configure the mailserver and the corresponding user credentials. This is done by appending the following line to /etc/mail/authinfo using your favorite text editor.

AuthInfo:your_mail_server.your_domain.tld "U:your_username" "P:your_password"

You have to replace the dummy information.

Updating the sendmail configuration

This step updates the sendmail configuration to include the previously added server credentials. Add the following lines in the file /etc/mail/sendmail.mc after the “FEATURE(`access_db’, , `skip’)dnl” directive.

FEATURE(`authinfo',`hash /etc/mail/authinfo')
define(`SMART_HOST', `your_mail_server.your_domain.tld')

Replace the hostname!

Saving the configuration

Use the following commands to save the configuration. After that you should be able to send mails through PHP.

cd /etc/mail/
sudo makemap hash authinfo < authinfo
sudo m4 sendmail.mc >sendmail.cf
sudo /etc/init.d/sendmail restart