Once upon a time, before email existed, we used to write letters, memos, and postcards. Letters and memos could be kept private in regular or inter-office envelopes, but postcards were different. They could be–and often were–read by anyone who happened on them.

I was first introduced to electronic communication via a weird DEC intranet system that allowed us to send what we now know as email only to people in the one company. I hated it–much preferred my IBM Correcting Selectric. But not long after, I had my first AOL email account, which I fell in love with the first time I found and booked cheap airline tickets through Sabre, which had been previously available only to travel agents.

But I digress.

As email became a “thing,” we were in a weird situation. The people who knew all about it–the IT geeks–weren’t, as a general rule, especially fond of communicating with people live. Training us to use the wonderous new technology we were being presented with wasn’t their forté. So I read manuals. Lots and lots and lots of manuals.

There weren’t many guidelines for using electronic communication back then, but there was one simple rule we learned early on: The Postcard Rule.

Simply put, this said, “Don’t put anything in an electronic message that you wouldn’t write on a postcard.” That is, if it’s private or secret, don’t put it in an email. Because you don’t know where it’s going to end up or who’s going to read it.

These days, it seems like this rule has gotten lost.

I get that sometimes it’s not practical, but in terms of marketing, it applies more than ever. Are there (more) secure forms of electronic communication? Sure, of course. And some are more private than others. But for any public figure (Authors! I mean you!), it’s worth assuming that anything you post, especially on Facebook, is going to be seen by everyone. And because the Internet never forgets, I mean anyone forever.

I’m not saying you shouldn’t share opinions, even controversial ones. But it’s a good idea to understand the repercussions of doing so. And The Postcard Rule will serve you well in this, as will understanding privacy settings and how to use the telephone…and stamps.