You can tell if a message has been delivered by looking at the icon next to the message on the Archive tab. If it has a green arrow, that means the message was delivered to your mail server.
You can click on the icon to get more details about the delivery, including the date & time it was delivered and the message ID to help you locate it in your servers log files.
If the icon does not have a green arrow, the message could not be delivered for some reason.
In that case, we will keep trying to deliver the message for up to 5 days. You can click on the icon to get more details about the failure, including the error response from your mail server.