When you start using the ONEmail, 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 ONEmail, you will be the first, and those IP addresses will be clean.
In order to get the best possible delivery, the following are a few points:
1) Try to keep the FROM and the "Sender" of your messages the same on ONEmail as they were on your previous system
2) Start sending to a smaller list of "high engagers." These are people who frequently opened or clicked on e-mail in your old mail system. Upload them into ONEmail first and send them your regular messages. This is called "warming" your IP addresses. These people will typically continue to open your messages, which will train the spam filters that your new IP addresses are sending wanted e-mail.
3) Make sure that you have valid SPF and DKIM records configured before sending your first mail. Contact your ONEcount support team for the correct info. You can test both SPF and DKIM records through https://mxtoolbox.com.
4) Your open rates dip for as much as two weeks after you start sending messages through the new IP addresses.
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.