When you send your e-mail campaigns, you will naturally get some messages that bounce back to you, and some will not even be delivered. Sometimes for good reasons, like an old and now invalid address, or the recipient's mailbox is full. However, some are legitimate email addresses that you can send to from your inbox just fine, but when you include them in your email campaign, they bounce back or do not get delivered.
This is because some mail servers at the larger ISPs, like sbcglobal, earthlink, comcast, peoplepc, yahoo, gmail, aol, etc., are continuously added and removing mail servers that are suspect, many times unjustifiably blacklisted, or blocked, because there is some suspicion that spam is coming from them.
We take every measure possible to make sure that our own mail servers are not blacklisted on any ISP. However, there isn't really any way for us to know if and when it may happen.
If you are hosting liveSite on your own web host, the same issues can occur.
What you need to do is to open the bounce back email messages, and view the message header (your email client will let you do this), and inspect the contents for the reason that the email was rejected. In the header will be the contact information for the ISP so you can contact them and remove your mail server from their list.
If you realize that some of your email recipients aren't getting your email, and they are all using the same email service (like gmail for example), then you will need to contact them.
Getting off of their "blacklist" is usually a simple web form to complete or and email that you have to send to them with your original message, so they can determine it is not spam, and they will begin to allow your email messages to pass through to their customers in about 24 hours.