You can use a dedicated mail file to store a copy and cull any NDRs but there will never be a guarantee they received it. I've had users inadvertently set up rules that automatically delete all incoming mail. We use a hybrid approach of mailing from dedicated mail files, using bcc to a central mail-in database with a retention period of 30 days, and an external account. We also use the bcc on all user generated mail from systems with workflow approval. When we get the call that the email wasn't received we just forward it from the central depository. 99 times out of 100 when they insist to know what happened we restore it from their trash. That anomalous 1 time we tell them to check the personal email account for the address stored in the originators address book.