Email marketer here, B2B. I don’t have an exact open rate decline percent, but I know we obviously aim for emails to hit the main inbox. The drop off from not landing there is substantial. Because I’m in B2B there are times customers can’t get our emails due to their corporate IT teams having stricter-than-average regulations on inbox placement. We tell them to whitelist our sending aliases, and if necessary we pass our sending IP address to their IT teams for advanced whitelisting.