Invites going to Spam in hotmail.

The reason e-mails go to spam does not have to do with the invite module. It has to do with your local e-mail server configuration as it relates to Hotmail, Yahoo, Google/Gmail, and any of the other sites.

Generally this means, at a minimum, reading all of their documentation and doing things like:

1) Configuring SPF records
2) Configuring DKIM records
3) Ensuring your content in the messages meets their guidelines
4) Sending from a mail server that has proper forward and reverse domain names that match (both A and PTR)

Each ISP has a page for e-mail server admins that explains the best way to get your mail delivered. Hotmail has one, and it's best to start with their mail server admin page and then also look into Microsoft's SenderID program.

As you can tell I have spent quite a bit of time on this issue. The best thing I can suggest to save you a LOT of time is to sign up for an account at www.authsmtp.com and configure all of your outbound mail relaying to go through their site. They maintain a mail relay server that has a high trust score with the major ISPs. I literally spent weeks on deliverability studies, called many paid delivery/relay services, and this is the only thing that is cheap and easy that I could find.

I'm not the invite module owner, but I am going to set this to 'won't fix.' Needs work is generally only applicable if there is a patch included and it needs additional work from the community.