Another reason is that many email service providers use a variety of tools to reduce email spam. These tools often try to detect that an email is originating from the location it claims to be originating from.
Please have a look at this post to solve the mailing issue at your website: http://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-not-sending-email-issue/
To start using Mailing for MicrojobEngine, you will need MjE Mailing instead of ET Mailing.
Also, there are bunch of 3rd party plugins that could work with EngineThemes: