>>>Hi,
>>>
>>>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?
>>
>>This is one of the most frequent issues reported by clients. It's usually caused when the recipient's mail server has marked the sender as a spammer and is routing the email to a spam folder or just blocking it.
>
>I hadn't seen Al's reply.
>His makes sense.

Agreed, a sending domain being blocklisted by a recipient domain is a common problem. There are several reasons why this may happen:

- Serious, persistent e-mail misconfiguration at the sending domain
- A computer at the sending domain getting infected by malware and used as a spambot. This requires killing the spambot and taking manual action to ask the blocklists to remove the block
- Guilt by association: If a mail server at xxx.xxx.xxx.123 is sending spam from imaspammer.net, blocklists may choose to block:
- any other domains that send mail using the xxx.xxx.xxx.123 mail server
- any other mail servers on that /24 subnet xxx.xxx.xxx.000 to xxx.xxx.xxx.254
- all mail sent by that entire ISP. This usually happens only for small, disreputable ISPs, not for large, generally reputable ones

If there are mail flow problems it never hurts to check if the sending domain is on any blocklists. One way to check is https://mxtoolbox.com/blacklists.aspx .

However, from the initial problem description this case doesn't sound like a blocklist issue.