A site visitor asks…
Is it an invasion of privacy to email someone because you can see their email address in a forwarded email sent from a mutual acquaintance?
Yes, yes it is!
Whomever sent that forwarded email should have redacted/removed all of their contact’s email addresses from the forward. I also have to wonder about the motives of forwarding emails of others to strangers…
But even before that, the original Sender should have used the BCc: field for addresses being sent to contacts that do not know each-other. (I have an article about the Email Etiquette involved in Forwarding and using the BCc: field.)
Dual Responsibility
These simple efforts by the original Sender (BCc: use) and the Forwarder (redacting) would have prevented any email addresses from being exposed to strangers. By not protecting these addresses they were the first to invade the privacy of their contacts. They caused the public display of email addresses to strangers.
Because they did not take these steps in no way provides tacit permission for anyone to email folks they don’t know.
Strangers or Mutual Acquaintances
If the only thing you have in common is that you both know the Sender or Forwarder but not each other, it is not recommended. The issue here is you have an email address that was not provided to you by its owner. If you are emailing about something you can commercially gain from you definitely should not email this “acquaintance.” That’s called spamming.
But let’s say you just want to reach out — that is part of being online, in groups and communities. I’ve met a bunch of nice folks who did just that. In this case you have two options. The preferred approach would be to ask the intermediary friend if they wouldn’t mind formally introducing you by email. Another option is to start the email with an reminder greeting about how/when you “met” mentioning the intermediary contact and a “hope you don’t mind if I email you…” type of statement.
That said, don’t be surprised if they do not appreciate getting an email from someone they don’t know regardless of who you both know in common!