The 21st century is no less challenging. Shifting consumer fashions and financial crises in places where port has always sold well have crimped earnings and cast a cloud over the trade's future. That is forcing producers to step outside their comfort zone and explore distant new markets.
Twelve million fewer bottles were sold in 2010 compared with 2000, according to the Association of Port Wine Companies, an industry group.
"The trade is clearly going through a difficult time," said Symington, 55, who describes himself as Portuguese "born and bred."
About 60 miles downriver from Symington's riverside house at Pinhao, there's a glimpse of where part of the solution could lie.
By the docks in Gaia, where the Douro meets the Atlantic and where sailing ships once departed heavy with wine barrels, two dozen Brazilian tourists chatter at a port tasting. They have just been on a tour of a port "lodge" - a warehouse containing thick 19th-century ledgers where wine slowly matures in aged vats made of oak, mahogany, and chestnut.
Aparecida Gilioli, a tour operator from Curitiba in Parana state, reckons the social aspirations of booming Brazil's swelling middle class will broaden the market there for top-drawer goods such as port.
"It's a sophisticated drink. It's chic," she said.
Buckingham Palace usually serves port at state banquets, and it was on the menu when Queen Elizabeth II hosted President Obama in May. That kind of patronage lends port distinction.
Part of the problem is that about 80 percent of production has traditionally been swallowed up by five main markets - France, the Netherlands, Portugal, Belgium, and the United States. Britain, the historic main market for port, ranks sixth these days.
The worst financial crisis in decades has helped depress sales in those countries. And the high-calorie drink has become less fashionable as consumers watch their waistline.
In the last century, Symington Family Estates exported port to about 20 countries. Now, it is selling in more than 80. The new destinations include cash-rich emerging markets such as Brazil, Russia, China, and Angola, which Portugal formerly ruled.
Producers are also pushing port in gimmicky new cocktails, such as white port with tonic water, a twist of lemon, and ice, that might appeal to a younger crowd. It's a novelty the trade's forebears might have found sacrilegious.
Symington, though, says it's an inevitable development. "We have to de-formalize port," Symington said.
British exporters built up the local wine trade after Britain went to war with France in 1678, losing access to French wines. The story goes that the British added grape brandy to the Douro wine to prevent it from spoiling at sea. That made it 20 percent proof - almost twice as potent as table wine - and especially sweet by halting fermentation while the wine was still fruity.
The Douro Valley is one of Europe's smartest addresses for wine and one of the world's oldest demarcated wine regions, dating from a 1756 law.
Its steep slopes, slate terrain, and sunny summers provide ideal conditions.
The vineyard estates, called quintas, are reached by narrow, winding roads that trace the contours of the Rio Douro.
During the fall grape harvest, grape-pickers move slowly along the rows of vines that ripple uphill from the broad river. Starting just before the sun comes up over the surrounding high ridges, they dip their hands into the bushy vine leaves that are turning auburn and snip off heavy bunches of grapes that are sweet as jam.
About 30,000 farmers along the valley use generations of know-how to produce wine. They rely on the lump sum from the harvest to help see them through the year.
Said vineyard worker Maria Augusta, 46: Wine "is our daily bread."