Mail server and WebSocket monitoring for the quiet failures
Check that your mail server greets and answers, and that your WebSocket endpoint accepts connections, without sending an email or a message.
Why it works this way
Some failures leave the website looking fine. Receipts and password resets stop arriving, or the live feed on a dashboard freezes while every page still loads, and customers notice long before anyone on the team does.
A mail server monitor connects with STARTTLS, implicit TLS or no encryption, waits for the greeting, says EHLO and reads the reply. It never signs in, sends mail or touches a mailbox. Port 25 cannot be reached from the checks, so point it at 587 or 465.
A WebSocket monitor opens a ws:// or wss:// connection and passes once the server accepts the upgrade, then closes without sending anything. It notices a live feed has gone while the rest of the site still answers.
What it costs
Those capabilities and everything they depend on come to $5/mo.
Read next
Build the plan around it.
Add what you need, drop what you do not, and read the total before you buy.