How do I know when the mail I have sent is received?
You won't unless you add an option to the e-mail to request a read
receipt AND the recipient has configured their e-mail client to enable
replying to read requests or configured it to prompt when there is a
read receipt request by an e-mail message and that recipient decides to
send a reply. If the user gets the prompt and decides to not send a
reply or if they configure to ignore all read receipt requests then you
won't get one. It is the *recipient's* decision whether or not they
respond to your request for a read receipt.
The delivery receipt request is handled by the recipient's mail server,
NOT by the recipient's e-mail client. This only tells you that it got
to the recipient's domain, not that the recipient ever saw it. Also,
not many mail servers bother to respond to requests for delivery
receipts (it is a waste of their resources).
