Google doesn't make public its spam detection rules (for good reason), so we can't say exactly why your emails would be marked as spam. But, commonly, emails get marked as spam when they include certain types of content (such as auto-generated content), don't have a proper SPF record, don't use DKIM, have other users of the email service mark similar content as spam, are sent from an IP address with a poor reputation, are sent from an IP address that looks dynamic, and many other factors.
Google would be able to provide you more information on why your emails, specifically, might be marked as spam. But, I don't know if they are willing to do so.
Certainly, SPF is a good first step, and having your users add you to their contact book and mark the messages as wrongly flagged would be helpful to you in teaching their system to treat them differently. (What you don't want is for users to check the spam folder and simply click the activation link in the message without signaling to Google that it's not spam.)
In terms of Google saying that "Lots of messages from
edge100x@nuclearfallout.net are spam", that is erroneous. It is likely due to two main factors: One, spammers/phishers using it as a return address (they cull public pages for email addresses and then use them both as destination for some and sender for others); and our forums emails being identified as spam without users correcting Google about it. (I will update that board email address to something else, since it shouldn't be using my email anyway.)
