Their FAQ states their system is configured to allow a maximum of 50 recipients per message, though they recommend you keep it to 10:
http://authsmtp.com/faqs/faq-20.html
If you contact their support team, they can answer any questions regarding limits. The few times I've had to interact with them they were very knowledgeable and helpful.
SPF is not enough. You need to set up domainkeys as well. see my post at http://www.kai-mai.com/node/156 for details.
You absolutely need to stop any open relay on your server. Once your server gets onto a black list, it's hard to get off it.
The other solution is to pay some provider to outsource your email delivery. Check http://www.emaildiscussions.com/
You shouldn't rely on gmail's IMAP server to deliver emails because it has daily limits.
SPAM flagging will have a lot to do with content, and some to do with your source IP (IME). Sharing a server (and source IP) with a bunch of other people doesn't help your odds.
Owning and controlling the server, the DNS records (forward AND reverse), plus sending a few test messages to test accounts, has always seemed to work best for me personally.
You can only send so many with a free account, but I believe you can send a lot more with their apps for domains product (or whatever it's called)... just a thought.