Emails not delivered

If emails you are sending are not being delivered, or you are not receiving emails sent to you from a specific sender, the sender may receive a bounce-back email from their mail server explaining why the message was rejected. In this case, the bounce-back message should include details explaining why the recipient's mail server rejected the message.

When no bounce-back message is received, the email might not have actually been sent. If the email is still in the sender's outbox, the problem is likely that their mail client is set to "work offline" or is having an error connecting to their mail server.

When an email appears to have been sent on the senders side but is not received and no error is reported, the message was likely detected as spam, or as having malicious content, and filtered out. In this case, the recipient may find the email in their Spam/Junk folder, and the headers in that email source should explain why it was detected as spam. Otherwise, the mail provider's IT support could check their email logs to track what happened to the email.