When you start using the ONEcount Email Engine, you will get a pair of "clean" ip addresses for sending mail. This means that they have no sender score. If you used a third-party ESP previously, your mail was sent from a pool of 12 to 48 IP addresses that are used to send a lot of bulk mail from a lot of companies, and so you have a shared sender score. This could be good or bad.
When you start sending from ONEcount, you will be the first, and those IP addresses will be clean.
In order to get the best possible delivery, following are a few points:
1) Try to keep the FROM and the "Sender" of your messages the same on ONEcount as they were on your previous system
2) Make sure that you have valid SPF records configured before sending your first mail
3) Your open rates will dip for about two weeks after you start sending messages through the new IP addresses.
4) To maximize your score, send your most-opened e-mails out through ONEcount for at least two weeks.
Some publishers worry about the dip, so they send sponsored e-mails through ONEcount for the first two weeks. If these don't have high open rates, you wind up with a bad sender score.
Instead, send your messages with the highest open rates. This tells ReturnPath and the major ISPs that messages coming from these IP addresses are valuable and it will get you a higher score.