poMMo is free open source mailing list management software. It's written in PHP and pretty basic but does give a decent management interface. However, it's no longer maintained. There is a GitHub repository from a guy who's trying to continue developing it sporadically so please contribute there if you can.

Some problems:

When installing, make sure /cache is writable.

After installation (assuming it was successful) test sending email by going to support/tests/mailing.test.php. You should see the message:

Initial spawn success. Respawn success. Spawning Works!

If instead you see:
Error message: "Initial Spawn Failed! You must correct this before poMMo can send mailings."

Then you have some problem with poMMo reaching your own web server. For example, if your web server is being password protected with basic auth, you'll need to hack the file inc/classes/mailctl.php and find:

// to allow for basic .htaccess http authentication,
//   uncomment and fill in the following;
// $out .= "Authorization: Basic " . base64_encode('username:password')."\r\n";

You need to set the username and password (replace username with an actual .htpasswd username and password with the password) then remove the '//' before each line.

If you see:
Initial Spawn Failed (test file not written)! You must correct this before poMMo can send mailings.
then you have some problem with your cache directory. You can edit the file to see what path/file it's trying to write to.

If you see:
Spawning Failed. Codes did not match.

then I feel sorry for you.