MAILER-DAEMON@●●.co.jp

みたいなエラーが頻繁に帰ってきます。原因は何でしょうか？

Notification from mailsystem

<EXPIRE NOTIFICATION>

Your 2 messages were removed by the mailsystem, because the duration
of keeping message expired. Please note that message duration in your
mailbox is set to 100 days.

↑こんなメッセージも帰ってきます。？？？？