The "dropped" email response indicates that a previous email that's been sent to an email address has failed, and that our mail server is no longer attempting to send to that particular address.
A few ways to ensure this doesn't happen is to check the email address to make sure it is spelt correctly, to check the size of any documents that may be attached to the email (we recommend trying not to send documents that are 15Mb or higher, as it can slow the ability to send the email which may result in it timing out / failing), or check your default message or notes that are connected to a quote or invoice and make sure there is nothing that may trigger a spam block.
