There's a number of reasons why an email send might fail - including some of the following:
- The address contains a space
- The address does not contain an @ symbol or contains 2 or more @ symbols
- The address ends with a .
Generally our system does not allow invalid addresses to be subscribed into a list or imported, however they can still be entered under certain circumstances, such as:
- When entering contacts using an older versions of the API
- When converting list fields
To address the issue, we recommend performing an Advanced Search and identifying contacts that meet the above criteria, and then deactivating, correcting or removing them from your list so that the send goes through.