In their latest whitepaper "Warning Signs That Your Client is Spamming", MailChimp lists the most common reasons they end up having to shut down an agency’s client for too many spam complaints:

• Old lists – If your client has been collecting email addresses on their website for years, and this is their first email campaign to the list, there will be people who don’t remember the client (“Who the heck are you, and how’d you get my email address!?!?”) These people will report you for spamming.
• Spam traps – Some ISPs take very old email addresses that they assume aren’t being used anymore, and they post them to public websites. Then they wait for spam-bots to scrape them, and spam them. As soon as they get spam to one of these “spam trap” addresses, they block the spammer. This is why you never send to a list more than a year or two old. It’s also why you should never buy an email list, and why you should never scrape emails off of websites. The effect of hitting a spam trap is devastating and fast.
• Tradeshow lists – When people attend a tradeshow, they usually buy their tickets online. They submit their email address. The tradeshow host then gives their email address to the companies who exhibit at the show. Companies can theoretically use this list to find prospects who plan to attend the show, and reach out to them. That’s fine, so long as the communication is one-to-one. But if they send an email campaign to the entire list, it’s spam, and they will get reported for it.
• Outlook address book dumps – This one’s extremely common with small businesses who don’t have big fancy customer databases. They just manage everything in their Microsoft Outlook Address Book. The problem is their address book doesn’t let them export a list of “only my customers” or “only people who opted-in for email marketing.” It exports everybody, including “grandma, that dude I met at a tradeshow 5 years ago, my ISP tech support that I emailed 2 years ago, and my ex-wife.” These people will report you for spam. But it’s not limited to small businesses. You may tell your client, “Okay, we’re prepping the big email campaign now. I’m going to need your customer email list.” Your client will then ask their entire company sales team, to “Give me your lists of customers by close of business, so we can get our exciting email newsletter out!” Guess what that sales team is going to do. They’re going to dump their entire address book out and send it over. They’re not going to spend the time to sort out opt-ins vs. non-opt-ins.
• Salesforce dumps – This is very similar to the “Address Book Dump” above, but at least you have some sort of classification (theoretically) of email lists. Be on the lookout for clients who dump all their different lists into one big one. Ask them if they combined their prospects list, leads Warning list, qualified leads list, customer list, and subscriber lists, then tell them how dangerous that is.
• Purchased lists – It’s a real no-brainer that purchasing 30 million emails from some seedy offshore company is a stupid idea. The thing is, most clients buy lists from local networking organizations, or tradeshows, or publications they advertise in. They sound innocent, and totally legit. And sometimes, the intent of the list seller is to let you send one-to-one communications (not spam). But the reality is that most people buy those lists to send unsolicited bulk email. So ask your client if the list was purchased. If it was purchased, then whoever sold them the list needs to send the bulk email. Or, the client needs to send totally different emails, one at a time. Unacceptable responses to this include, “But this list is all legit” and “But this list is all opt-in” and “But this list was very, very expensive” and “But this list came from a very reputable industry source that everybody knows.”
• Organization lists – Your client may be a member of a realtors’ organization, or a local business group. Organizations will often give you their membership directory whenever you join. This is for one-to-one networking. Not mass-subscribing them all to email marketing. The most vicious spam complaints can come from these lists, because very often your client’s competitors will be members of the same organization.
• Chambers of Commerce lists – When you see a small/new company that has an inexplicably large email list, it’s probably from their local chamber. Again, the lists they give out are for one-to-one networking with other business owners. Not mass email marketing.
• Lists from their previous ESP – If you’re helping a client switch from one email service provider to another, make sure they’re exporting the latest clean version of their subscriber list. Some clients will mistakenly export their entire list of subscribers (even those who previously unsubscribed, or bounced).

Download the whitepaper here.