This is the first time I've run into this and but it won't be the last. The client has been using this shared host for some time now and it is only recently that it was brought to their attention that their contact form wasn't working as expected.
The client is running the latest release of the version 4.5 branch of the TYPO3 CMS. When the form is submitted it seems to be working but no email is sent to the intended recipient. So I checked the CMS configuration and it was set to use SMTP transport and the port(25), user and password were correct.