Mails were working fine and no settings have changed on the smtp server, receive an error code 550
Did the issue occur in SharePoint 2010 or SharePoint 2013?
Did you configure the outgoing mail with office 365?
If the issue exists in SharePoint 2013, then you could check if the outgoing mail is configured correctly according to the article below.
Configure outgoing email for a SharePoint 2013 farm.