A Network Administrator Migrated the Data for the Corporate Email

Multiple Choice

A network administrator migrated the data for the corporate email service from one provider to another. All DNS records were successfully changed and verified. However, many hours later, users are still receiving new emails on the old provider's server. Which of the following should the administrator perform NEXT to troubleshoot the issue?

A) Add a secondary MX server to the DNS records.
B) Remove old SPF records from the DNS host.
C) Create a DMARC record that includes the old server.
D) Check the TTL value on the MX records.