If I send an email to teacher, I get an error email

> This is the mail system at host ms12.shibaura-it.ac.jp.
>
> I'm sorry to have to inform you that your message could not
> be delivered to one or more recipients. It's attached below.
>
> For further assistance, please send mail to postmaster.
>
> If you do so, please include this problem report. You can
> delete your own text from the attached returned message.
>
> The mail system

If you receive the above error mail from the Mail Delivery System, it means that the recipient's mail box is full. If teacher is using Gmail, it is likely that they are also reading mail.

If you have any problems, please contact us.