Test Mail Server for Inbound TLS
Here’s how to test if a mail server supports TLS using a windows pc:
- nslookup
> set q=mx
> google.com - 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 - > exit
- telnet google.com.s9a1.psmtp.com 25
- After connected type:
ehlo google.com - 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 year | Downtime per month | Downtime per week |
| 90% | 36.5 days | 72 hours | 16.8 hours |
| 99% | 3.65 days | 7.20 hours | 1.68 hours |
| 99.9% (“three nines”) | 8.76 hours | 43.2 minutes | 10.1 minutes |
| 99.99% (“four nines”) | 52.6 minutes | 4.32 minutes | 1.01 minutes |
| 99.999% (“five nines”) | 5.26 minutes | 25.9 seconds | 6.05 seconds |
| 99.9999% (“six nines”) | 31.5 seconds | 2.59 seconds | 0.605 seconds |
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