The below example message indicates the email was sent with a "Delivery Receipt" enabled.
Your message was successfully delivered to the destination(s)
listed below. If the message was delivered to mailbox you will
receive no further notifications. Otherwise you may still receive
notifications of mail delivery errors from other systems.
The mail system
<RECIPIENT@DOMAIN>: delivery via vpmout: delivered via vpmout service
A "Delivery Receipt" is intended to inform the original sender when a email is delivered to a mail system. While this message was requested by the sender, it can be confusing.
If the original sender does not want to receive this message, then delivery receipts will need to be disabled in the mail client.
Below are links that may assist you with locating this setting in your mail client.