How to extend Zimbra MailboxListener for an Extension
You can read details about how to write an Extension for Zimbra in my previous post: https://senoritadeveloper.medium.com/zimbra-extension-development-6d2ebe91e737
You should extend your class from https://github.com/Zimbra/zm-mailbox/blob/develop/store/src/java/com/zimbra/cs/mailbox/MailboxListener.java
Operation List: https://github.com/Zimbra/zm-mailbox/blob/develop/store/src/java/com/zimbra/cs/mailbox/MailboxOperation.java
Item Types List: https://github.com/Zimbra/zm-mailbox/blob/develop/store/src/java/com/zimbra/cs/mailbox/MailItem.java
Example classes which extends MailboxListener class:
https://forums.zimbra.org/viewtopic.php?t=3141
https://forums.zimbra.org/viewtopic.php?t=3277
https://forums.zimbra.org/viewtopic.php?t=3421
Happy Coding!