No route to host

Hi folks,

I've set up mailbox.org for a client that doesn't have a website, they just use their domain for email. Unfortunately, mail sent to the mailbox.org accounts is not arriving. The error I'm seeing is:

####

This is the mail system at host mout-b-201.mailbox.org.

I'm sorry to have to inform you that your message could not

be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can

delete your own text from the attached returned message.

The mail system

<email goes here, I removed it for privacy>: connect to domain.name[192.0.2.1]:25: No route to

host

####

Anybody aware of what needs to happen to correct this?