WP Moo sees the change on its next check, and the problem goes away by itself. A new record can take a few hours to reach everyone.
Your domain lost its mail records
Email to your addresses bounces until the records are back. Put back the MX records from your mail provider's help pages. They often get lost when the domain moves to another DNS provider, or when a hosting change resets its records.
Does the domain no longer take email, on purpose? Remove MX from the record types of DNS & mail in Checks & pages.
Your domain has no mail records
Should the domain receive email? Add the MX records your mail provider gives you.
If it shouldn't, remove MX from the record types of DNS & mail in Checks & pages. Then keep an SPF record of v=spf1 -all and a DMARC record with p=reject, so nobody can send email in its name.
No valid SPF record
Keep exactly one TXT record starting with v=spf1 that names every service sending your email. Your mail provider's help pages give the exact record.
Got two SPF records? That counts as none, so merge them into one. For example, a site that sends through both Google Workspace and Mailchimp's Mandrill gets v=spf1 include:_spf.google.com include:spf.mandrillapp.com ~all.
No valid DMARC record
Add a TXT record named _dmarc with v=DMARC1; p=none to start with. That changes nothing about your email yet, but it's valid, and mail servers see that you've set it up. Once your email passes SPF or DKIM everywhere, you can move to p=quarantine or p=reject.