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Help with a 5.1.10 bounce message

Hi guys,


We are using Office365 smtp to send outbound emails outside our organization.

There is however a some percentage (not all) of failed deliveries which are returned as: (see below).


Most of the rejected emails are gmail and hotmail.

I personally checked some of them and they exist for sure.


Any hints with this issue?

This is the error output I get:


Your message to couldn't be delivered.

someaddress wasn't found at


Status code: 550 5.1.1 Status code: 550 5.1.1 This error occurs because the sender sent a message to an email address outside of Office 365, but the address is incorrect or doesn't exist at the destination domain. The error is reported by the recipient domain's email server, but most often it must be fixed by the person who sent the message. If the steps in the How to Fix It section above don't fix the problem, and you're the email admin for the recipient, try one or more of the following: