You are the Microsoft Office 365 administrator for an accounting firm.
An accounting user at your firm reports that a customer received a Non-Delivery Report (NDR) when the customer sent an email to the accounting user. Others in the organization are receiving and sending email without any issues.
What is likely causing this issue?
A. There is an MX record error.
B. The Federated Server is down.
C. The mailbox size limit is reached.
D. The SMTP server is backed up.
