What MX records do
MX (mail exchanger) records tell the rest of the internet which servers accept email for your domain, and in which order. The lowest priority number is tried first. If your MX records point to the wrong place, or to a name that does not resolve, mail is delayed or lost.
What the SMTP test checks
- Connection and greeting: whether the mail server answers on port 25 and how quickly.
- STARTTLS: whether mail to you can be encrypted in transit, and which TLS version is used.
- Certificate: who issued the mail server certificate and when it expires.
- Reverse DNS (PTR): whether the mail server IP has a host name that resolves back to the same IP. Gmail and Outlook expect this from any server that sends mail.
Common MX setups
Google Workspace uses smtp.google.com (older setups list five aspmx servers). Microsoft 365 uses yourdomain-com.mail.protection.outlook.com. Zoho uses mx.zoho.in, mx2 and mx3. cPanel hosting usually points MX at mail.yourdomain. If you changed email provider and mail still arrives at the old one, the MX records are the first thing to check.