Its pretty unlikely that your IP is being used by others to make SMTP connections to the server since it is a relatively small server and only a handful of users use satellite Internet the odds are it is directly related to you.

Ratelimiting generally occurs for the following reasons:

1. Incoming SMTP connections that violate RFCs (this is usually spammers and broken MTA)
2. Incoming SMTP connections that do not send QUIT
3. Incoming SMTP connections that have recently matched an RBL
4. Incoming SMTP connections that have attacked the server
5. Hosts which transport messages with spam score from SpamAssassinTM is greater than 20.0.

So I would start crossing items off the list. Like number 5 for example. Was the email you sent before you were Ratelimited Spammy by nature? Lots of HTML, images, spammy words? If not, then move on down the line.

I can tell you it is not number 3 because I looked up your IP at Spamhaus and Spamcop and you are not listed. So 5 and 3 are gone.

1 and 2 are your most likely causes.

Also regarding the 500 email per hour thing, you are confusing that with the number of emails you can send off from the server in an hour. You can't possible send 500 emails in an hour with a standard email program, you would use a script for that and that script would be unaffected by ratelimit since the script's IP is on the server versus the IP which is getting ratelimited, which is your IP.

I'd try another email program or update the one you have to see if it fixes the problem as a start.