These emails are returned to the server due to being rejected by the recipient's email server. Causes of this could be:
Recipient's address is invalid or the domain name doesn't exist or recipient is unknown - any future emails to this address will be "dropped" - the server will not attempt to deliver them
Recipient's server rejected the email due to: previously marked an email from you as spam, inbox is full, etc.
If the server has already attempted to deliver a previous email to this address and it was invalid, the server will not try again for future emails. This will result in the status "dropped". You will want to verify the email address with the client, make changes, and try again!