How MX routing works
Mail Transfer Agents use MX records to discover where to deliver messages for a domain. Each record contains a preference number and a mail-server hostname. Lower numbers are preferred. Equal preference values may be used to distribute delivery attempts. The destination hostname should resolve to usable address records.
Interpreting missing and null MX records
An explicit null MX, written as priority 0 with a dot as its target, declares that a domain does not accept email. With no MX record, SMTP can fall back to address records under the mail protocol rules; therefore an empty MX answer alone does not prove whether email works. This checker resolves addresses for up to four MX hosts to bound server work.
What to check after an email migration
Compare the answer with the exact MX values supplied by your email provider. Remove obsolete records only after confirming the provider’s migration instructions. Check DNS hosting and TTLs if old answers remain. SPF, DKIM and DMARC are separate DNS policies; use the DNS checker for their TXT records. Mailbox passwords, SMTP connectivity and spam filtering are not tested here.