If your not sure what a domain key is, you can read more up on them here http://en.wikipedia....Identified_MailI we have added DKIM to all servers and it's now a part of the standard setup.
This should help resolve any rejected mail from some providers, however there are many reasons mail can be rejected.
A few good measures
1. Always provide an unsubscribe option on any email that is sent out from an email generator or forum script.
2. Make sure you have proper SPF records at your DNS Registrar (make sure to add the server.domain.com as an allowed sender)
3. Monitor rejection emails, never send mail from "noreply@domain.com" this will flag you faster then you can say spam. When you get a rejected email, follow up on it at least once with each provider, if you are being blocked for hitting a filter, find that filter source and ask to be removed.
For the most part, it's a war, we fight to block spam, and we fight to send good valid emails. If you have any further questions let me know.