Your message couldn't be delivered because the recipient's email system wasn't available when we tried to talk to it. It could be a temporary condition that may clear up soon.

Try to resend your message later. If it still doesn’t work, contact the recipient by some other means (by phone for example) and ask them to tell their email admin that their email system is having troubles accepting messages. The recipient’s email admin is the only one who can fix this.

For more information see Status code 4.4.7 in Outlook.com and Office 365.