You have an Exchange 2010 email system and you get a kick back email message that says an email has not been delivered. This can be a frustrating experience when such a basic transaction is rejected. The question is exactly what happen and how to resolve it.
It is important to understand how email flows to understand exactly where the breakdown occurred. When you send an email from Outlook, it gets placed in a queue on Exchange. Exchange 2010 will then start the process of finding and communicating with the other receiving party to deliver this message.
In a simplistic world if you have that email address correct deliver should be a sure thing. No questions asked. However nowadays there are many layers to this email journey. First of all emails have to travel through your ISP (Internet Service Provider) system. Because of concerns with SPAM many ISP have a checkered policy on emails. For example some ISP prevents any flow of email unless you reach out to them and specifically ask to open all email ports. These sorts of problems should be easily to detect if no emails are going out and if you can carefully do some port testing to see where emails are stopping.
The other more probably culprit in an undelivered email is the recipients SPAM filter system. Many times you can tell from the kick back email you received that states clearly that the SPAM filter has block this email due to various reasons. Sometimes it is not very clear and you may have to sort through the message carefully to figure out what the reasons are. You can look over previous blog for more information.
In conjunction with the SPAM filter, you may have winded up on a blacklist. This can happen if your system is sending out SPAM relay because of a compromised mailbox or an open port. It can also happen if you have an infected machine that is delivering mass emails and causing your Exchange to be the bad actor. You can check if you are on a blacklist here.
Another reason SPAM filter may block your email is that your emailing IP doesn’t have a properly reverse DNS lookup value. You should following this blog to make sure you are setup correctly.
Other unlikely but possible reasons maybe, that the recipient accidentally put you on a their blacklist; I’ve seen the ISP get on a blacklist; Perhaps the content you are sending may have a link that is flagged; email address is auto cached incorrectly in outlook; and lastly but more common then you think the recipients emails system is temporarily down.