Where do I find the MX record when troubleshooting external email delivery?

A client is reporting that their internal email works perfectly, but they haven't received an external message all morning. I suspect their domain configuration was changed recently and need to verify the mail exchange destination. Where can this be verified?

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OPTIONS:
A. DNS
B. VLAN
C. DHCP
D. SMTP

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Testing Exam: CompTIA A+ Core 1 (220-1201)
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