Sending mails from third parties to third parties

I'm developing an application that will send emails from third parties to third parties. Every email is one of a kind (think about a forum with email notifications) but obviously that system can be abused by spammers.

I'll do my best to ensure accounts are real, verifying addresses, adding 开发者_C百科captchas and so on, but I still expect spam to go through and be marked as spam at Hotmail, Gmail and so on. What's the best approach for doing this? Is there any documentation of the dos and don'ts to be a nice email citizen. Can Google, Microsoft, Yahoo, etc. notify me when a mail my system sent was marked as spam so I act accordingly?

Antispam systems won't notify the sender when the mail is dropped - if they did, spammers could abuse this mechanism to find holes in the antispam implementation.

As for "pretending to be a sender from a different domain" - this will raise a number of spam flags, as it's the same mechanism that's also abused by spammers. Amongst other things, the PTR, DKIM and similar mechanisms for your mail won't match (and you can't fix that, as you're not in control of the sender's domain).

What I'd recommend is to send the e-mail as yourself, but set Reply-to header:

From: robot@yoursite.example.com
Reply-To: piskvor@example.org
Subject: Piskvor@example.org has sent you a message

This way, you're not spoofing the origin of the message, yet the replies will go to the alleged sender.

Oh, and do maintain and read the postmaster and abuse mailboxes of your domain - if there's a spam complaint, it'll be likely adressed to one of those.