Have set up contact form in Joomla 1.5

On EMBC network so use their smtp settings all works fine providing email from field is on a domain hosted by them.

Any other accounts eg @sky.com @hotmail.co.uk it will not relay

Thought about using other smtp server external to EMBC but firewall block ports

Senders email could go in comment box but how do I hide the from email address input box and have it default to an EMBC address

Any ideas?????