Absolutely. I do like to remind people that it's the Inter-Net, an interconnection of the essentially independent and private networks of many, many different organisations (including employers, ISPs, universities and so on and so forth). Although there's governance of a kind, each organisation's IT department gets to make decisions about what it's prepared to accept.
So, any IT department staffed by people who don't know about forums and mailing lists, or don't (rightly or wrongly) see them as a priority, may well set its criteria for spam detection in such a way as to reject a proportion of that mail.
Of course, in reality, everyone has to stick within certain norms, or find no mail arriving at all(!), but there is plenty of scope for the loss of mail that's much more important to me than to my ISP, in the name of defending my fellow-customers from spam.