An email bounce signifies the non-delivery of your email messages. This failure may originate from the recipient’s mail server for a number of reasons:

• The email address is invalid 
• The email addresses don’t exist
• The mailbox was full (the user is over their quota) 
• The server was down 
• The message was too large for the recipient’s inbox
• Some recipients have marked your mail as spam
• The recipient's organization IT department needs to whitelist emails from mobilize.io (mark them as valid)

Usually, the bounce message will give you important information to help you identify the reason for the email delivery failure. This includes the following:

• The time and date the message bounced
• The mail server that bounced it
• The RFC code and reason for the bounce 

It is highly recommended sharing the error that generated the bounce error with your member, so they or an IT manager can help fix the problem. 

Note: If you Unbounce an email address that has not been fixed yet, it can cause your group email address to be marked as spam and hurt your reputation score.