Somebody hacked into my contacts list and keeps sending spam to the.  Someone suggested changing my password but to no effect.  I get  loads of messages similar to this and lots from organisations like Ebay.  " From: "Mail Delivery System" <*** Email address is removed for privacy ***>
To: <*** Email address is removed for privacy ***>
Subject: Delivery Status Notification (Failure)
Date: 16 January 2016 16:50

The following message to <*** Email address is removed for privacy ***> was undeliverable.
The reason for the problem:
5.1.0 - Unknown address error 550-'5.1.2 <*** Email address is removed for privacy ***>: Recipient address rejected: Domain not found' 
And I have no clue who the recipients are.  How do I stop this.  The first occurence was while I was away for a few weeks. Yours Hrid