Especially if you are using a hotmail address:
Diagnostic-Code: SMTP; 550 5.7.1 Unfortunately, messages from [xxx.xxx.xxx.xxx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [DB8EUR06FT022.eop-eur06.prod.protection.outlook.com]
Fuck You Microsoft !