Custom context is the way to go in a typical setup as such
Have you tried hyla fax included in the elastix package for providing an e-mail-to-fax functionality than using winprint?
Yeah we are receiving faxes this way (fax-to-email) but the email-to-fax setup seemed a bit convoluted. I had to also consider my users. Most aren't tech savvy so I went with the path of least resistance. If you have info any info that outlines this setup I'd love to take a look at it.