When an email address bounces permanently (it fails), if the failed email address matches the email address on the lead, the lead's permission to email will be set to 'No'.

When carrying out bulk emails, any leads which have the permission to email set to "No" will not be sent an email.

However, if you were to manually send an email from the lead this would be allowed.

If you have a correct or different email address, you can change the permission to email back to "Yes".

Simply click Edit on the lead. Click Contact Preferences and click Permission to Email "Yes". You will then be able to email this customer.

It is possible to Search for leads that have the permission set to No. simply click on + Add Filter from Search leads. Select Permission to Email Is the value No.