Wrong "Undelivered Mail Returned to Sender" message

[pere]

When somebody sends an email to accounts in my server, the sender gets the message "Undelivered Mail Returned to Sender" but this is false and the receiver gets the mail. I ask, can you kindly guide me on where this problem should be found? The additional message says "host
hotmail-com.olc.protection.outlook.com[104.47.18.97] said: 550 5.7.1
Unfortunately, messages from [207.154.227.131] 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.
[AM6EUR05FT017.eop-eur05.prod.protection.outlook.com] (in reply to MAIL
FROM command)"
Thank you!