This is a fragment of a screen shot from my email software, PoliteMail. Although the company allows the option of using their mail servers, that has led to higher undeliverable messages for me. The regular email server for the account (a free Windows Live account) lets me send about 200 indentical messages a day, and by breaking the newsletter emailing into waves, I find that I'm better able to handle the information flow and notice how it's going. For a small database like ours (less than 1000 but shortly to double in size), this method allows me to give personal attention to any problem.
An email bounce is a member with whom we've lost the connection. If I see a formally active member whose email starts bouncing, I often have a phone number and I can get an updated email address from them.