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!