After upgrading from Ubuntu 12 to Ubuntu 16 I discovered an error for Drupal website sending e-mails using Mailsystem + Postmark
here is the error code:
Error: Call to undefined function curl_init() in Mail_Postmark->send() (line 322 of /srv/www/reinisfischer.com/public_html/sites/all/libraries/postmark-php/Postmark.php).
Luckily there is an easy solution, installing missing curl:
sudo apt-get install php-curl
Hope, that helps!