Contact Us form not sending emails

I'm on a server that's not really fond of the standard Mail function. I've set it up through the opencart backend to send using SMTP and sending through the backend works fine.

Now the problem is, that when trying to send from the contact form, I get the error:

"Notice: Error: MAIL FROM not accepted from server! in xxxxxxxxx\system\library\mail.php on line 298"

I read many posts regarding this problem and haven't found a solution yet. Anyone got an idea what could be causing this?

If you use the smtp function you will have to use a mailbox at the same server.
many mail providers dont allow to send mail throug their mail servers from outside their network to prvent automated spam.

so you have to create a mail adres for your domain.
this can be done in either cpanel or direct admin.
now your server can access the mailbox on the same server as your website and mail will be sent

That's not the case as I can send from the admin interface with my current settings.