SpamPort has an optional technology called Greylisting. This can be enabled on a per-domain base.
With greylisting turned on, we temporary reject mails from unknown IP's. We tell them that the service is not available currently, try again in a minute.
Usually, normal mailservers will try again in 1 to 10 minutes. However, mass (spam)mailing servers are usually too busy queueing up e-mail and will therefor not try again.
This technology can therefor help to reduce spam to your mailbox.
The downside to this technology, is that some mailservers (including Gmail) will take longer to try again. Therefor the delay might be 45 minutes or even hours.
After a new IP has successfully retried, we keep the IP/domain combination in our whitelist for a few days, so the next time the mails will go through directly. If a few IP's from the range have been successfully whitelisted, the whole range will be accepted without delay. Some mailservers (Outlook, for example) have been whitelisted by us and will always go through without a delay.