Awesome that certain countries keep changing their place names insisting that it's so foreigners pronounce it "correctly" by using an irrational sequence of letters.
The Wikipedia article about the mangrove region contains the word “Sundarbans” 198 times, including the title, while the word “Sunderbans” occurs 20 times. It doesn't even mention the version with E as an alternative spelling. Is it a common misspelling or an accepted alternative?