The only gmail specific part seems to be about the body parsing. As different clients/services sends bodies in different formats, they are all using multipart,

The only gmail specific part seems to be about the body parsing. As different clients/services sends bodies in different formats, they are all using multipart, but some use multipart/alternative, some use multipart/mixed and so on. Seems author skipped that part ("In any case this looks like multipart bodies in HTTP. I don't want to deal with that so I'm just going to stop here."), so they tested receiving emails from just one specific client (gmail). Anything else would probably not render correctly, especially if they use other encodings which would make the message unreadable without extra parsing (think koi8-ru)