Secondary groups (***@mail2.adm.kyoto-u.ac.jp) are powered by Google Groups.

When you send an email from a Google Group address and it bounces on its way, you cannot receive a bounce message (*1).

*1 A bounce message is an automated message from an email system, informing the sender of a previous message that the message has not been delivered or some other delivery problem occurred.

You CAN receive a bounce message:

• when you sent an email from a Google Group address on the web version of KUMail (Gmail)
• when you sent an email from a Google Group address with your email client via Google's SMTP server
• when you sent an email from a system and set a personal (individual) email address for the envelope sender (*2)

You CANNOT receive a bounce message:

• when you sent an email from a Google Group address with your email client via a non-Google STMP server
  Workaround: Specify a Google server for SMTP.
• when you sent an email from a system and set your group address for the envelope sender (*2)
  Workaround: Contact the system admin and ask them to change the envelope sender.

*2 An envelope sender is where computers should respond in the case of bounce messages or errors, whereas a sender (From) address is where people should respond. It is usually specified by Return-Path in the email header.

Groups administrator FAQ (Google Workspace Admin Help)

Under "Are there any types of messages I can't send to a group?", Google says "You can't send or forward non-delivery receipts (NDRs), also called "bounce messages", to a group. Messages that resemble NDRs are also not permitted."