Can you tell us which application and version you are using to fax from your MAC?
Could you first start by re applying the 10.4.11 combo update from here http://support.apple.com/kb/TA24901?viewlocale=en_US after it has run coul dyou reboot your mac and see if it will respond to faxes.
Let us know what happens.
Some people have had success reapplying hte update to get the faxing working again, The other option would be to use a third party software program to handle your faxing for you, However if the combo update works for free it would be best as it is intigrated.
If you prefer to go with another application let me know
Let us know what happens when you get it done.
As always the users are the best source of information when it comes to bugs and fixes, Also people like me who work on corporate networks every day come accross a lot of these issues and av chance to fix them long before the makers even find out there is an issue.
Cn you tell me if you are sending a fax from the print command and also if you are using a cover sheet.
Can you try turning off the coversheet and see if it will go.
Let me know
Let me check a few thingsand hiopefully I will have something solidfor you in the AM.