Test Mail Server for Inbound TLS

Here’s how to test if a mail server supports TLS using a windows pc:

  1. nslookup
    > set q=mx
    > google.com
  2. The results:
    google.com   MX preference = 100, mail exchanger = google.com.s9a1.psmtp.com
    google.com   MX preference = 300, mail exchanger = google.com.s9b1.psmtp.com
    google.com   MX preference = 200, mail exchanger = google.com.s9a2.psmtp.com
    google.com   MX preference = 400, mail exchanger = google.com.s9b2.psmtp.com
  3. > exit
  4. telnet google.com.s9a1.psmtp.com 25
  5. After connected type:
    ehlo google.com
  6. If you see this in the output, the mail server supports inbound TLS communication:
    250-STARTTLS

Understanding the “Nines of Availability”

If you’ve spent any amount of time in the tech field you’ve probably heard of the “Nines of Availability”. Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously.

Availability %Downtime per yearDowntime per monthDowntime per week
90%36.5 days72 hours16.8 hours
99%3.65 days7.20 hours1.68 hours
99.9% (“three nines”)8.76 hours43.2 minutes10.1 minutes
99.99% (“four nines”)52.6 minutes4.32 minutes1.01 minutes
99.999% (“five nines”)5.26 minutes25.9 seconds6.05 seconds
99.9999% (“six nines”)31.5 seconds2.59 seconds0.605 seconds