Contact form on web server

Hi!,
This is my first post to this forum.
I’ve been working with my webpage in localhost and i’ve just uploaded it to the host via ftp. Everything seems to work fine but the goddamn contact form. I have no clue what can be causing problems because in localhost works perfectly.
I am using Contact Form 7 and a plugin called WP SMTP to configure the emails. The error message are surrounded with a red borderline and says it is a mail server trouble. Does anybody know what could be the difference that causes this problem in my web host and not in localhost?
Thanks!