Firstly check your mail server configuration on the following site:
https://www.mail-tester.com/

You should test it by sending email from roundcube and email client software like outlook.

Next, lets check your mail server reputation and rDNS/DNS match
http://www.senderbase.org

And as last, check your mail server black-listing
http://mxtoolbox.com/blacklists.aspx

When sending test emails you can check your mail server logs by monitoring it live using the following command

tail -f /var/log/maillog