Possible Causes: 

1. The most common cause of a bounce is an invalid email address subscribed to a list.  In this case, the owner should “delete” the email address from the list.  More details below.
2. The list server misinterprets automated replies as a bounce.   While most auto replies are interpreted correctly, we’ve seen this bug in the software and have reported it to the development team, www.sympa.org.

The Setting:

Mail list owners can change the notification setting at:  Admin > Edit List Config > Bounces   This setting defines the bounce rate (% of subscribers bouncing) required before the sever notifies the list owner bounces exceed the set values.  The default values are:  warn=30%, halt=50%.   More info at:  https://sympadocs.web.illinois.edu/campus-mailing-lists-users-guide/list-owner-docs/the-list-owner-settings/edit-list-config/bounces/  

Resetting the Bounce Rate:  

To reset the bounce the bounce rate go to:  Admin > Bounces.   Review the email list, select the checkbox next to the email address you want to “reset” or “delete” from the list. 

• If you have a large list, change the view to “500”. 
• To select all email addresses click the checkbox at the top of the check box column.