Open and click tracking
On the Pro plan, email tells you whether it arrived and what happened next. This is the one part of Human RM that involves a server, so it is worth explaining exactly what it does.
How it works
Two mechanisms, both of them the industry standard. An invisible image is added to the end of the message; when a mail client loads that image, the message is recorded as opened. Links in the body are rewritten to pass through a redirect, so a click can be counted before the reader is sent on to where they were going.
Links in your signature are deliberately left alone. Your booking page, your website, your book link — those stay exactly as you wrote them, because a signature is part of your identity rather than part of the campaign.
What is recorded, and what is not
The service stores that a particular message was opened, when, and which of its links were clicked. That is the whole list.
It does not store your contacts. Not their names, not their addresses, not their numbers, and no copy of what you wrote. Human RM has no account system and no address book on any server, which is exactly why there is nothing there to lose. The privacy page says this at greater length.
If the tracking service is unreachable, your email still sends
Why opens are approximate
Treat opens as a signal and not a fact. Apple Mail Privacy Protection loads that image whether or not anyone read the message, which inflates the number. Plenty of other clients block images entirely, which deflates it. Someone who read your message in a preview pane with images off is recorded as never having opened it, and someone who never looked at it may be recorded as having done so twice.
Clicks are much closer to the truth, because a click is a deliberate act. If you are trying to work out who to follow up with, use clicks.
Switching it off
Tracking can be turned off in settings, and then nothing is added to your messages at all — no pixel, no rewritten links, no record anywhere. Some people leave it off for personal sends and on for organisational ones, which is a reasonable line to draw. Texts are never tracked under any setting.