The best way to customise add and unsubscribe membership notification emailss

We use hook_mail_alter to alter the message (or disable it from being sent out all together).

Target which message you want to alter with $message['key'] and then edit $message['body'][].

You can get the group with \Drupal::routeMatch()->getParameter('group');.

Thanks @Pixiecat.
The hook_mail_alter is a good way to handle it. My only issue is that it doesn’t work consistently; sometimes the default message goes through without the alter. Do you know what could be the issue? I’m filtering the messages by key before altering.