@Sabatino said: "When microsoft starts receiving emails from a new mail server, or rather from a new ip even from an existing mail server, you are guilty until proven guilty. "
Very much agree. We've found Google just as unpleasant, until the first 30-45 days has passed without complaints.
In December, we brought a customer onboard that generates traffic of about 2.5 million messages per month. (All of it legit person-to-person. They're just a big customer.) The first month+ was miserable delivering to mailboxes hosted at MS (outlook.com) and Gmail, and traffic was coming from our IP address space that is squeaky clean.
The irony is, the MS and Gmail networks are also among the largest sources of spam we get. They are Nazi-like in their inbound filtering, but overly liberal in what they let out... and they're "too big to block".