Quite often I get asked why someone's email is not delivered. I usually run through a series of questions since there could be a number of reasons for why an email was not delivered. Figuring out where the problem lies, either the sending or receiving side is the first step. Below are the questions I normally ask to help resolve the issue.
Is there a way you can contact your recipient to confirm delivery? Via IM or phone? Confirm the recipient's mailbox is not full.
The best way to resolve is to have direct communication from sender to recipient. It will aid in all the above questions and fix the issue quickly.
When did you start getting this error?
This helps to identify new problems versus recurring issues. And also narrows down options for the below questions.
Can you send emails to other people besides the one you are getting an error back from?
I ask this to see if there is an issue with the sender's email account or server.
Were you able to send to him before this or is this the first time you're sending to this address?
Say Mary is sending and email to Bob and gets an error back. This helps identify if it could possibly be a bad email address that Bob gave Mary or if Mary may have typed the email address incorrectly.
How long after sending did you get this email below?
Knowing this, we can send test messages to confirm the error and run more tests.
If your emails were getting to your recipient before, did your email that failed to deliver contain a large attachment?
This helps identify mailboxes that may be full or delivery restrictions on the sending or receiving end. If regular text emails are getting delivered, it could very well be mailbox restrictions.
Did I leave any questions out? How do you troubleshoot your email problems?