php mail function using From header

[updatedoffers.com66]

hey,
whenever i use the mail function and specify a

From: user@domain.com

in the header, the email is not sent, it does not throw an error, it simply does not send, if i do not use the from header field then the email is sent from what i guess is the default email

updated@web8.vital.x10hosting

i have tried using the From header and the -f command line and neither will send the email from the email i specify
is there any reason for this? everything used to work as expected before and now its not sending anything...
can i work around this? is it a new spam prevention method? i am also using gmail as my mx server, im not sure if that would matter

thanks,
dave
UpdatedOffers.com