We will never spam, but we may be blamed of spamming if , for example someone uses password reminder many times.. What can be done about it?

We want to serve millions without being marked a spammer.

I think short term solution would be using different server and domain when sending emails out automatically; but this doesn't help in the long run.. any ideas..

You will only be a spammer if you send a mail to someone who doesn't request it. If someone uses a password reminder multiple times they have made the request and therefore you are not spamming them.

Yes, but spam detectors don't work that way..