When you send an email you can get back a “Delivery Status Notification” message. The subject may vary.
99% of the time that message is a delivery failure notification (email doesn’t exist, mailbox full, account disables, spam, …).
So, why is the Newsletter plugin reporting zero errors on the statistics page?
This is something important to understand and we can use the post office example. When you want to send a letter, you write it, put it into an envelope with the destination address, and bring it to the post office.
They accept the letter and the “delivery” starts. For you, the letter has been sent. But:
- the letter can be lost
- the letter can be rejected by the receiver
- the letter can have an invalid address
All those reasons for delivery failure are notified days later. And sometimes they aren’t notified at all.
With email things get even worse:
- the notification is an email (the Delivery Status Notification) that is not sent to the software that originally sent it (for example the Newsletter plugin)
- the email can be dropped by your provider (your local post office) or can be dropped by the recipient email provider (the remote post office)
- the error notification couldn’t contain references to the original message, but only information about the recipient (for example that it’s not valid)
Even if a notification seems to arrive a few seconds after the message has been sent it happens anyway after the message has been sent. The post office (your provider) does not refuse immediately the message.
Those are called “bounces”.
Sometimes an error is reported during the initial sending phase (when you are at the post office with your letter…) and it can be assigned to that specific message.
Other kinds of errors are when the provider does not work properly (the post office is closed). The latter case is managed by suspending the delivery (that can be restarted).
Where are those delivery status notifications sent back?
- they could be sent back to the sender’s email address (the one you see on the From field in the email client)
- the could be sent back to the “return path” address if specified (the Newsletter plugin has this setting available on the main options)
- they could be sent back to an address “forced” by your provider because your provider wants to intercept them (for many valid reasons)
Anyway, the sending software cannot know about them and a bounce management system is required to deal with those delivery errors.
By the way, dealing with bounces is extremely important since it affects your domain reliability as emails origin.