SMTP connect failed

My form responses are stored in Perch, but aren't reaching my inbox. The Perch test e-mail gives an 'SMTP connection failed' error. So I'm checking if all of my settings are correct, but I can't find the problem. I've got the latest version of Perch, I'm running PHP7, I've double checked the SMTP-settings and tried using another SMTP-server. I can't think of any more I can check. Can someone give me some advice?

Perch: 2.8.34, PHP: 7.0.13, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: 241ae00989d1995ffcbbf63d579943635faf9972 $, with PDO
Server OS: Linux, cgi-fcgi
Installed apps: content (2.8.34), assets (2.8.34), categories (2.8.34), perch_blog (5.0), perch_forms (1.8.3)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', 'perch_forms', );
PERCH_LOGINPATH: /perch
PERCH_PATH: /home/flexiuy104/domains/apz.flexiflyer.nl/public_html/perch
PERCH_CORE: /home/flexiuy104/domains/apz.flexiflyer.nl/public_html/perch/core
PERCH_RESFILEPATH: /home/flexiuy104/domains/apz.flexiflyer.nl/public_html/perch/resources
Image manipulation: GD
PHP limits: Max upload 64M, Max POST 64M, Memory: 128M, Total max file upload: 64M
F1: 6a33f95eca3667f9e0c39bf5ca2980fe
Resource folder writeable: Yes
DOCUMENT_ROOT: /home/flexiuy104/domains/apz.flexiflyer.nl/public_html
HTTP_HOST: apz.flexiflyer.nl
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php