Historically I have been using Uptime Robot a lot, but in practice they are only good for HTTP/HTTPS uptime checking and TCP connection checking. SMTP checking requires more than just a valid TCP check which they can’t, so I took a look at hetrixtools.com.
They advertise being forever free when using a limit number of Uptime/Blacklist endpoints as per
[Wayback/Archive] Free Uptime Monitor & Blacklist Monitor- HetrixTools
- 15 Uptime Monitors • 1 Minute Interval
- 32 Blacklist Monitors • Checked Everyday
FREE Forever!
Unlike many other free uptime checking tools, their interval is better.
A thing to consider is that, like many online-first companies, they do not provide company details on either of these pages which you have to agree with when signing on for their service:
The Wiert Corner - irregular stream of stuff 