What this means is that Ultimate Form Mail is having trouble accessing the email resources on your server. The code is "working" but when it tells your server "send the email" your server says: "Nope, you didn't ask me the right way."

But there's something you can try. You can try different ways to ask the server to send out the email.

First, open up controls.php and do a search for 'smtp'. That will bring you to a line where you can choose four different methods of sending emails from your server. Depending on your host, you'll have to try each one until one works.

If one doesn't work, you have to ask your hosting company what's needed to send emails from the server... smtp? mail()

You can tell your hosting company
"I have a script that sends out emails from the server. It has four ways it can send it out, smtp, mail function, sendmail, qmail. I've tried all four and they don't work. Is there something special I need to know to give code on my server the ability to send out an email?"

They will probably reply with something brilliant like:
We don't support scripting issues.
Contact the vendor.

That's the typical response.

In the past, it's turned out that when the hosting company actually takes time to look at the issue, that there's something amiss with the exim settings on the server, or some other issue preventing emails from going out. Other times it turns out to be that a username and password is required for smtp, and the host doesn't allow any other method of sending out emails... which is fine, Ultimate Form Mail can send any username, host, password to access the smtp. Just modify controls.php

Bottom line, if you're getting the 'thank you' page, then Ultimate Form Mail is "working". But if your server is restrictive on how to send the email, then you need to configure Ultimate Form Mail so that it asks your server the right way.