You closed a Ticket sending an e-mail for the customer; if the e-mail returns by 'Mailer Daemon' or customer replies the Ticket it gets reopened.
Reproducing the Issue
1) Close a Ticket sending an e-mail for the customer
2) Notice the Ticket getting reopened if the e-mail is not correct (causing Mailer Daemon) or Customer replies
This is the system standard behavior. If the Customer has some question/concern about the closed Ticket he should be able to respond and keep the Ticket open. The same applies for Mailer Daemon scenario where the Customer even did not receive the final answer.
This is the system standard behavior.
KBA , LOD-CRM-EMP , Employee , How To