Recently a client offered to buy the emails I had received prior to a specific date: not just addresses, but the actual messages.
The client did not specifically ask for private or business correspondence, and said that this was for some data collection project. The only condition given was that they had to be written in English (I am not an English native speaker).
Would that be legal?
Without a data privacy document provided by the client that exactly describes what they want to do with the data, that would be an absolute no-go for me. Additionally for me, they would need to sign a non-disclosure agreement with a very high penalty in case of non-conformance.
But even then, I would still refuse this offer, particularly because you where asked to hand over business e-mails. I suspect they contain sensitive information from both you and your (other) clients. If these clients find out what you did, I would not be surprised if they immediately terminate any contract with you and charge you for disclosing their trade secrets. These probable consequences will be much more expensive than the bucks you get from the deal, even though 40 cents per mail is a pretty good price.
So while I don't think that this practice would be criminally prosecutable, there's a big risk that it would run against the principle of good faith and therefore make you liable for damages in a civil lawsuit. One of the fundamental civil laws in switzerland says: (ZGB, Article 2)
Every person must act in good faith in the exercise of his or her rights and in the performance of his or her obligations.
The manifest abuse of a right is not protected by law.
UWG (The Unfair Competition Act) Article 5 says it explicitly:
Unlauter handelt insbesondere, wer [...] ein ihm anvertrautes Arbeitsergebnis wie Offerten, Berechnungen oder Pläne unbefugt verwertet;
An unfair action performs in particular whoever uses any work results such as offers, calculations or plans without authorization. (my translation)
Some jurisdictions also have specific laws that forbid passing on trade secrets, for example:
If the gdpr applies to you (you didn't say where you live or work from or who your clients are), this would be illegal.
You said you are working and not a private person with their friends list on a piece of paper, you electronically process data of your (potential) business partners, so if you are in countries where the GDPR was made into local laws, they would apply to you.
Since this is not a government agency and you are not mandated by law and it is not required to run your business, unless you have their consent, it is illegal to sell (or otherwise give away) personal data of your clients, customers or even employees.
Please note that while you may live in a country where this is legal, your customers would still be in their right to never use your services again if they find out, and/or sue you based on their contracts with you, how they are worded, what they guarantee and where the court is located you agreed to use for disputes.
However, since you tagged it that way, this has nothing to do with copyright. Unless these emails contain something that actually has a copyright (lets say someone attached a photo or a chapter of a book they wrote for proof reading, or maybe made their meeting announcement in the form of a neat little poem), pure business correspondence is not copyrightable. Neither you nor anybody else can have a copyright to "Yes, I'll take 40 at $3 each" or "Lets have a meeting tomorrow at 2 PM" or "This is too much, lets see if we can do it in under two weeks!". You cannot copyright a normal everyday conversation (unless it's used as a piece of art, which your correspondence certainly isn't).
An email is a literary work and the author owns the copyright. That means you can’t copy it without permission.
While it’s arguable that you have a licence to use the work for the purpose for which it was sent, you definitely don’t have a licence to sell them to an unknown third-party. Doing so also doesn’t fall into any statutory defences like fair use.
I have located at least one case where the judge rules that dissemination of a business email was copyright infringement.
