What SMTP diagnostics do
Works like MXToolbox “smtp:host”: resolve MX for a domain, or A for a host/IP; on port 25 grab the banner, EHLO, probe open relay, check PTR and sample DNSBL.
Fix red rows first (connect, missing PTR, open relay, blacklist). Warnings (banner mismatch, no STARTTLS) can hurt deliverability without meaning the server is broken.
Examples
- Host
mail.example.comDirect SMTP host (MXToolbox smtp:…)- Domain
example.comSMTP test against primary MX
Reading the results
Checklist is Status / Test / Result, with an SMTP session transcript below. MX and extra ports are secondary; the decision is the port 25 diagnostic.
Open Relay should be OK (external RCPT rejected). If accepted, close the relay immediately.