If your email is getting blocked as spam and getting that return message you may want to try and contact yours/there support to see when they will update and include new Firefox into their data base.

If you are receiving said emails from other sources and want to report tham as spam can get a free spamcop.net account or even donate 15bucks as will go for a year or more.

Not sure if this helps but this is what is happening to my best understanding.

Please let us know if this solved your issue or if need further assistance.

I am in contact with the organisation and will post when I get any useful information from them. Their spam system seems very complex and scores various elements of the e-mail. It doesn't like courtesy as 'Dear Sir' and 'Yours faithfully' both contributed to the spam score !