Recently we've been having a few issues with in that our ESP (Responsys) has been marking emails as sent successfully but they simply haven't turned up either in the junk folder or any other folder in the email client. This was happening for about 48 hours and the only way we could tell was by the fact that our open rate on the Outlook Domain ISP was less than 2% compared with our average of about 30% across AOL, Gmail and Yahoo.

Now, we've recently taken a look further back and can see that consistently we've had roughly 10% lower open rate on compared with AOL, Gmail and Yahoo. The trouble is that we don't know whether its because the emails aren't getting into the inbox or that our recipients are just less active. Is there anything else that might be causing this? Any ideas on how we might move forward? :-)

Thanks in advance!