You just hit the "send" button and your email campaign is launched. Then what happens? At this point, your ESP (ie. Email Service Provider-Constant Contact, Vertical Response, MailChimp, etc.) takes over. Hopefully, you've followed best practices for email marketing and have done the following:
You'll also find emails that didn't quite make it where they were supposed to go. An email bounces for several reasons. It can be blocked by a server, your recipients could be on vacation (lucky them), or some glitch occurred during transmission.
Any "non-existent" addresses should be reviewed for obvious typos (extra spaces, .com vs. .net, etc.). Remove these. Repeated attempts to email non-existent addresses will hurt your deliverability rates. They remain a subscriber and may affect your account status.
Source: Constant Contact
If you have a large list, consider exporting your bounces on a regular basis and cleaning up any recommended for removal. The "mailbox full" category generally means these are on the way to being non-existent. The "suspended" category, (used by Constant Contact), is a safety mechanism which lets you know that you've attempted to email a non-existent address more than once and will be placed on hold so that your deliverability rates are not affected.
Remember, a well-maintained list will provide more accurate results, and results are what will help determine your next campaign.
Liz Provo, Mass Marketing Resources.