Many servers do not have this enabled, so even if you select DSN option, it does depend on whether the server chooses to repond or not.

re :Tools...Options..Advanced ....marked Delivered Status Notification When sending messages always request a delivery status notification from teh server.

You could not have selected 'When sending messages always request a delivery status notification ' because this option does not exist in that location.

Tools > Options > Advanced > Return Receipts checkbox: 'When sending messages always request a return receipt' Return Receipts is not Delivery Status Notification.

Delivery Status Notification is mesage that server received email, assuming server has this enabled. Return Receipts is dependent upon the recipient and whatever software they use to read emails. It is not obligatory.

You can't rely upon Return Receipts because many email clients and webmail let the user decide whether to ignore return receipts, or to decide on a case by case basis whether to allow the return receipt. http://kb.mozillazine.org/Figuring_out_whether_the_recipient_read_your_message

check to see if 'Disposition-Notification-To': is included in the header. Create email as if to send to colleague or yourself at different email address. Use the 'Send Later' option to put email in 'Outbox'

Go to Outbox and select email click on 'More' and select 'View source' Do you see a header line: Disposition-Notification-To: email addess

Please report back on whether 'Disposition-Notification-To:' header is in email seen in 'Outbox'.