What do we usually do when we meet an unfamiliar word in a letter? We launch the browser, open the search engine site (or use the toolbar), make a request. I have written a small extension for the Mozilla Thunderbird email client that simplifies this process. It is enough to select a word (or phrase) in the letter and select "Find ... on the Internet" in the context menu. The search engine page opens with search results.
By default, the search is performed by Google. In the extension settings, you can choose another search engine (Yandex, Bing, AOL, Baidu).