What might be the reason for receiving a message that couldn't be delivered because the sending email address was not recognized as a valid sender, often suspected of sending spam, and what should be done to resolve it?
a) The email address is incorrect; contact the recipient for the correct address.
b) The email server is down; wait and try again later.
c) The email address is suspected of sending spam; contact the organization's email admin with the error message.
d) The email attachment is too large; reduce the size and resend.