I'm about to launch a service which offers email forwarding for a large number of domains.
Say you want people to be able to contact you using firstname.lastname@example.org, all emails sent to that address would be forwarded to email@example.com
We will have 3 servers with their own IPs, the servers will run Postfix and all domains will have abuse@ and postmaster@ addresses which will forward to our admins.
I don't want our IPs to get blocked, all we're doing is forwarding the received messages, but I don't really want to enable spam blocking on the forwarded mail (It's our users, not ours, it should be up to their onbound email provider to detect spam and put it in a 'spam' folder, the only option we have is to delete spam messages but if we start getting it wrong and people start getting upset that we're not delivering their emails we could get in trouble with them)
One option is frequently (maybe once a month) get new IPs for the servers so they should be clean (We can get them for about $2/year per IP so no huge cost to us, but we end up with a lot of 'soiled' ips lying around which our hosting company won't take back). Are there any other suggestions for what we can do so we don't get blocked by email providers?
We'll be forwarding between 15-20k emails a day to ~10k different accounts. We're currently using OpenSRS' email forwarding service but it's too expensive now we've got to this level of users