Heck, I'm sure you already know.........
The email entered during the opt-in process is invalid. People sometimes make mistakes when subscribing to mailing lists. Other times they deliberately enter an old email address out of concerns for spam or an increase in email volume. No matter what the reason, invalid email addresses are a common source of bounces. You can largely avoid this problem by using a double opt-in procedure before adding people to your list.
The sending IP address you are using has been blocked. Even if everything is fine with the message itself, the IP address you are using to send it could be blocked. This block can occur if you are suspected of spamming or email fraud or if your company has developed a bad reputation. You can avoid this problem by using quality email marketing techniques and never renting or buying prepackaged lists.
The email was blocked by the server (unable to relay). Emails can be blocked by the receiving server for a number of reasons. Many servers will block messages over a certain size, emails that use the wrong format and messages that appear to be spam. Mail servers can block your email also if the “From” address might not match an account on the email server. Reviewing your messages carefully, making your images as small as possible and following best practices can help you avoid this problem. Please note that mail server may block your email also for “technical” problems that you can’t have any control over, eg. “Out of memory”, “Connection timed out” and “Resources temporarily unavailable”:
The receiving server is overloaded or temporarily unavailable. Even the most robust server can only handle so much traffic. Sometimes an email will bounce due to an overloaded or unavailable server. This type of situation generates a soft bounce.
The receiving mailbox is full (Quota exceeded). Few email providers allow unlimited storage. When a user reaches their predetermined limit, all future emails will bounce. Many email systems will hold those bounced emails and attempt re-delivery for a period ranging from hours to days. If and when the recipient clears old messages and creates space, any cached emails will be delivered.
The email has been blocked by the recipient. Individuals can block emails from senders they no longer want to hear from. If that has happened to you, a bounce will be generated.
The recipient has activated an auto-responder or vacation reply. When employees go on vacation or will be unavailable, they can set up an auto-responder to tell contacts about it. If the email address you are trying to reach has an auto-responder, you will see a bounce, but the message may still be delivered after the auto-responder is turned off.
...also what email address are you trying to reach involving HLYK
