The only purpose of DSN "Delivery Status Notification" receipts is to let a sender know when the recipient's server received the message.

Note: that many servers are ignoring DSN requests partly as a measure to not auto respond to what may be spam.

"Return Receipt" when composing a message it will add the "Disposition-Notification-To: ..." header to the message that requests the recipients email client to send a reply and specifies who to send the reply to. However, you can't rely upon this because many email clients and webmail let the recipient decide whether to ignore return receipts, or to decide on a case by case basis whether to allow the return receipt. So the recipient might respond or might ignore the request.

Some servers may even remove the "Disposition-Notification-To: ..." header to remove the choice entirely so their users do not even see a 'Return Receipt' request. Sending Return Receipt will add additional load on their number of sent mail quotas.

So, whilst you can request DSN and/or Return Receipt, you cannot be guaranteed a reponse.

Thank you very much for your detailed response Toad Hall. That being the case do you know whether there is any way of tracking where emails have gone or perhaps got stuck?

If the recipient is using a POP account and the server spam filter auto put the email into Spam: Pop accounts will only download from server Inbox into pop mail account Inbox, so those emails will not be seen until the recipient logs on to their webmail account to check folders eg: Spam, Bulk Mail etc - other than Inbox.

You say that for one particular recipient, some emails get through but others do not. Check to see if you have more than one entry in your various address books just in case one of them has the wrong email address which might look similiar. Check to see if there is anything different in the content which could be flagging up a possible spam filter.