Why is it bad to send out a mailing list with one letter, filling all the recipients in BCC?

Question

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Somewhere right there on Habré I saw a mention of this, but no matter how I searched, I can not find it. At one time, when I worked in a company that had to send price lists to three hundred partners, I did just that - everyone was on fire in BCC, there were no problems (and I myself often received letters from partners where hundreds of addresses were openly filled in TO) . But probably since people use mailing scripts in separate letters, then this makes sense. So what?

I apologize in advance. I'm pretty sure this has already been discussed, but I couldn't find it.

Answer 1

I see one main problem: if everyone is written in bcc, then who will be in the "to" field? A person will receive a letter that is sent to no one knows where.
Writing addresses to a regular copy is bad because this way you can merge the addresses of your partners to competitors.
That is why people use mailing scripts: so that recipients receive a personalized letter, and other recipients cannot extract subscribers' addresses from it.

Answer 2

Privacy, of course. Vasya Ivanov will not be very happy to see himself in the mailing list for 100,500 users about the renewal of the Viagra course.