Webmail or PHP mail function both not sending email

I tried to send mail from my Web Application with my email function. Result shows success but no email was in inbox/spam in receiver's email account. I tried different approaches like smpt, simple mail() but no success. [I used WordPress and Codeigniter for my applications/sites.]

So, i tried to send mail from Webmail (using roundcube) like-wise shows success and is found in send list but no email received in receiver's inbox/spam.

I don't know what is causing this. Is there error in my Mailing code OR in my Hosting mail settings. Please help me.