You may have email addresses in your contact list that you can no longer reach. This leads to your emails not being delivered, because they cannot be received by your contact for some reason. The email address may no longer exist, may be misspelled, or the recipient's mailbox might be full. These are called bounced emails. It's pointless to keep these email addresses in your contact list. Once an address ends up on your bounce list, we filter it out before sending each mailing — so we recommend cleaning up your list.
You can find all invalid mail addresses on the Contacts dashboard via Marketing - Contacts. If you select a contact from this list, you will find the reason the mailings do not arrive under Contact information.
You can now replace this mail address, or delete the contact via Delete contact at the bottom of the page.