When either provider decides your small email server is sending spam (eg: sending an email with an attachment, or any kind of form email like a daily report) you won't get through to user inboxes, and instead you'll be routed to spam, or for Outlook.com hosted addresses they will accept mail from your server and send it to /dev/null. Gmail's process is bad, but Microsoft has decided to accept emails and throw them away (which is ridiculous).
