I received a mail with the subject "Undelivered Mail Returned to Sender".

You receive this email with the subject "Undelivered Mail Returned to Sender" when the email you sent was not delivered to the recipient due to some kind of trouble.
In many cases, returned mail is caused by incorrect email address. Please check the recipient's email address once again.

If the address is correct, try checking with the recipient by referring to the reason indicated in the returned mail

Where the reason is indicated:
Please check the text body, and try identifying the cause by referring to Causes by case.

- Below the line "The mail system".
- Below the line, "The following addresses had permanent fatal errors".

E.g., if the reason is located below the line "The mail system"
This is the mail system at host mail**.asahi-net.or.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 .
If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system
<******@asahinet.jp>: host
sbmx.asahi-net.or.jp[] said: 550 5.7.1
<******@asahinet.jp>: Recipient address rejected: Unknown user (in reply to RCPT TO command)

■Causes by case
●User unknown
●User not found
●Invalid User

This is a case when the mail was delivered to the recipient host, but the applicable user (mail address) was not found. Please check with the recipient.
If the returned mail is a reply to the received mail, the recipient's email address (or the return mail address) may be incorrect.

●Host unknown
●Host not found

This is a case when the sender host could not find the recipient host.
Please check the host code of recipient's email address (e.g., the part after the @. If the address is Asahi Net, host code refers to asahi-net.or.jp).
In another case, there may be failure in the sender or recipient host.

●The user(s) account is temporarily over quota
●Message size exceeds fixed maximum message size
●exceeded storage allocation

This is a case when the sent email volume is too big, or the recipient's mailbox has exceeded the limit.
Check the recipient's volume limit or reduce the size of your attachment accordingly. If you are sending from a mobile phone, try reducing the number of characters.

●Cannot send message for xx days (weeks)
●Unable to deliver mail

This is a case when the SMTP (outgoing mail) host server has been sending the mail for xx days (or weeks), but the recipient's host server did not receive the mail.
Please check if there is no failure or congestion in the recipient's server. There is also a case where the recipient's server does not exist.

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