1) Are you sending one connection with multiple e-mails per connection to the MX of comcast.net?
2) Are you sending one email per connection to the MX of comcast.net?
3) Are you sending individual e-mail to each person?
4) Are you sending a fake to and BCC everyone at once?
1 is better than 2
3 should be the standard
4 if you are, you deserve everything you get
Also, attachments such as PDFs are seem as advertisements by *MANY* type and kinds of spam classifiers. I'd suggest you include a short description of the recipe (or an HTML version of the recipe with no pictures) and then include a link to the location on your site and let the person download the PDF.
PDF isn't as bad as a BMP/JPG/PNG only in the e-mail.
This coming from experience sending millions of e-mail per year and seeing the after effects of PDF vs straight HTML with the plaintext version (plaintext is required as per the rfc governing that stuff, HTML is an option part) and just the plaintext versions.