Here's the back story.

If you haven't heard yet, Hotmail accounts were recently switched to the Outlook interface.

All of your subscribers with Hotmail addresses were offered the option to switch their addresses to Outlook addresses. They're also able to create multiple new email addresses in their accounts.

For email marketers, this means a few things.

All of these addresses point to the same inbox.

Anyone subscribed to your emails will still get them. Emails to their Hotmail addresses automatically forward to their Outlook addresses.

However, getting in the inbox doesn't mean staying in the inbox.

Your new Outlook subscribers can now use a "clean sweep" tool to clear all newsletters from their inbox.

If you've seen a dip in your open rates over the past few weeks, that's a likely explanation.

Here's what you can do.

It goes without saying that you'll want to keep sending as engaging, interesting and helpful emails as possible, so subscribers don't sweep your messages away.

But it's also important that you make sure you're testing emails for all your existing campaigns in Outlook.com - after all, it just went from 80 million users to over 400 million, and some of those new users are probably on your list.