>A customer reported a problem where some of the emails sent by my ASP.NET application are rejected. The error message is:
>

>The server response was: 4.4.62 Mail sent to the wrong Office 365
>

>
>I noticed that all emails that were rejected have a common domain. For example "@country.gov"
>
>I don't know how email servers work. But do you think the organization IT should whitelist this domain ("@country.gov") in the email server?

No. The problem is being caused by an incomplete configuration or misconfiguration of your customer's mail sending infrastructure. Here's what's happening when your app is sending a message:

1. The message has a "from" field, let's say it's SomeAddress@DLCustomer.com
2. The message is sent to AnotherAddress@country.gov
3. When the message is sent your app is using DLCustomer's mail sending infrastructure (you haven't specified the details, this can make a big difference)
4. country.gov's mail receiving infrastructure will perform various checks against the incoming mail message. Any missing or mis-configuration of DLCustomer's mail sending infrastructure will increase the spam score of the message and may cause it to be rejected
5. If country.gov's mail is received by M365/Exchange Online then there are additional checks. I haven't encountered the exact message you're seeing but you could get DLCustomer's tech support to go through https://docs.microsoft.com/en-us/exchange/troubleshoot/email-delivery/wrong-office-365-region-exo

It sounds like the "whitelisting" you're asking about would be to ask country.gov to whitelist mail from DLCustomer.com:

- No sysadmin in their right mind would do that to work around a misconfiguration at DLCustomer.com
- Even if the whitelisting was put in place, there's a lower level routing issue/rejection happening per the link above. The mail will not get to country.gov to be accepted or rejected until that specific issue is addressed