I have to ask my customers to add [email address removed] ? 

This is crazy, i can not ask every customer to add this so I can invoice them, I will just have to keep letting them know that it is probably in their SPAM folder. This is crap and if I can not figure out a better way to do it then I will have to find another software to use. 

Can I add a SPF TXT record to my domain host to get around this problem? I dont really know what that means but I was told this may work.