(E.G. TO: email@example.com;firstname.lastname@example.org;email@example.com)
If the recipient is using a program like Microsoft Outlook™, Microsoft Outlook Express™ or others, if the recipient right clicks on any name in the TO: field or CC: field, that person's email address is visible and available to be harvested and sold to the spammers or your competitors.
Some names are worth dollars, so there is plenty of incentive to sell the names.
This national foods company, who happens to drive pale yellow trucks and sells ice cream, delivers to me every two weeks. Their driver, trying to improve his commission, asked if I'd like to be on his email list to receive the following week's "Specials" so that I could order on-line. It sounded good to me, so I said yes. A couple of weeks later, the first email arrives. They have great specials, but what concerned me was... 164 addresses shown in the CC: field, mine being one of them.
These are bona fide, fully qualified customers! How much would this list be worth to you, if you were selling a competitive product in the same neighborhood? I'll bet I could have received a lot of free food from his competitors! Am I getting all of this diet products spam because someone stole his list and know that this company sell's premium ice cream? I'll never know!
Although not a total solution, when sending to multiple addresses, use the BCC: field. Only the recipients address shows on the recipients copy.