[Appleman]

I know OS X has a built in fax machine.
It works when it wants, and sometimes it doesn't want.
It sends, but it never gives you feedback that it actually is sent, and tries to send again and again.

I am wondering which program you are using to send/receive a fax.
Thanks.

[Big Mac]

pagesender: Full-featured fax software for Mac OS X

[milhouse]

I use trustfax. It is an email/online based faxing system. It has never failed to receive or deliver.

It negates the need for a phone line.

YMMV and good luck

[brettcamp]

Read about FaxCenter here: FaxCenter for OS X: All the Fax Software You May Ever Need.