Successful income investors need to look beyond the dividend yield. A juicy yield can be a good starting point for a long-term income portfolio, but it's just the beginning for further research. In some cases, extremely high dividend yields actually serve as warning signs for deeply troubled stocks and unsuccessful business models.

On that note, here are three generous dividend payers whose yields of at least 4% are tied to solid dividend policies and healthy cash flows. Read on to learn more about SK Telecom (NYSE:SKM), Retail Opportunity Investments (NASDAQ:ROIC), and Carnival (NYSE:CCL).

Several stacks of coins on a tabletop, next to an hourglass with lots of sand left to fall.

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Korean cash

SK Telecom is the largest wireless carrier in South Korea. It holds a 42% market share in a thriving industry where the second-largest name stops at a share of 26%.

Like many major telecommunications companies, SK Telecom generates a ton of cash from a stable set of subscribers. The company pays out the majority of that cash to shareholders in the form of dividends, leaving a comfortable margin of freedom to raise the payouts -- or to put its cash to use in the pursuit of further long-term growth. Over the trailing four quarters, SK Telecom paid out 68% of its free cash through dividend checks.

The company's dividend yield stands at a meaty 4.4% right now. That's relatively standard fare for a major phone company, but many investors also view this stock as a growth investment. You see, SK Telecom launched 5G wireless services in the spring of 2019, making it one of the first commercial-grade deployments of 5G technologies in the world.

The 5G service was launched with 250,000 5G customers signing up in the first month. That total swelled to 1.5 million accounts by the end of September, setting the stage for rapid adoption of 5G services and handsets in South Korea. Eighty-five percent of these new customers signed up for the top-tier unlimited service, which is good news for SK Telecom's profit margins.

This stock gives you a cash-backed dividend with room to grow, attached to a healthy market leader with a clear path toward expanding profit margins.

A profitable retail idea in 2019

Retail Opportunity Investments isn't a retailer in the traditional sense. Instead, it owns land and buildings in high-quality shopping centers anchored by popular grocery stores in densely populated markets along the West Coast. The company currently owns 88 shopping centers with a total area of 10.1 million square feet, and 97.7% of that retail space is on active leases right now.

As a real estate investment trust, or REIT, Retail Opportunity is required to pay out at least 90% of its taxable income to shareholders in the form of dividends. Falling short of that requirement would cancel the benefits of operating as a REIT and force the company to pay taxes on its operating profits. Instead, Retail Opportunity tends to pay dividends far beyond the REIT minimum. Trailing dividends currently stand at $0.80 per share, which is more than twice the company's earnings of $0.39 per share. The effective yield is 4.4%.

The secret to Retail Opportunity's success lies in its handpicked properties. This REIT doesn't invest in fixer-uppers or turnaround prospects. Shopping centers in this asset portfolio need the support of a thriving community with solid local economies. The retailers you find in the company's shopping centers tend to be household names that can make a long-term commitment to their leases.

That's another generous dividend yield backed by a fantastic cash machine. The REIT structure also allows Retail Opportunities to hold onto more of its profits, passing that benefit down to shareholders through the dividend policy.

Lock in this 4.4% yield while it lasts

Carnival Corporation is the parent company of Carnival Cruise Lines, a massive leisure travel company that operates over 100 cruise ships under 10 different cruise line brands. As a provider of luxury services, the company is sensitive to swings in the global economy. A lot of people skip going on expensive vacations when they might need their rainy-day cash reserves someday soon. The trade wars between China and the U.S. are weighing on Carnival's results, and the Brexit debacle is a common theme in Carnival's earnings calls.

So the company isn't doing great at the moment, and share prices have plunged in recent months. After a 20% price drop in 52 weeks, Carnival's shares now trade at a bargain-bin valuation of 10.3 times earnings. The same price trend also pushed the dividend yield higher, landing at 4.4%.

For a less robust company, these issues might be deal-breakers. But Carnival remains a fundamentally healthy giant of the cruise services sector, sporting a 15% profit margin and one of the healthiest balance sheets in the industry. And none of its rivals can match this company's economies of scale.

When the economy gets back on its feet again, Carnival should bounce back in a hurry. That will also lower the dividend yield very quickly, so the opportunity to lock in the current 4.4% dividend yield won't be around forever.