We found out that Yahoo and Hotmail (including Live) are automatically filing mail from Siafoo as junk. Even worse the Yahoo mail server(s) was returning deferred delivery errors, causing our mail server to retry sending the message a few times and then to give up. We attempted to contact Yahoo's support about these problems but (both times) we received a pseudo-automated reply with some links to help pages related to mail delivery.
In an attempt to remedy the situation we have implemented both DomainKeys Identified Mail(DKIM) (something of a Yahoo invention) and Sender Policy Framework (SPF) (something Microsoft likes) on our mail system. The result is that the delayed delivery errors have stopped allowing Siafoo e-mail to get to your junk folder faster, no doubt an improvement.
The problem as far as you are concerned is that any Siafoo invitations (or articles/snippets) you sent to friends and colleagues (and I am sure you've sent dozens if not hundreds and if you haven't... you should) with e-mail hosted by Yahoo and Hotmail probably ended up in their junk folder. So if you sent an invite or other siafoo based communication to someone and never got a response you might want to ask them to add siafoo.net to their respective safe lists. Or simply sent them whatever you wanted through personal e-mail.
The two workarounds for the mail delivery problems are to either switch to another email provider that has working junk filters (like gmail) or add siafoo.net to your safe list.