We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/using-spellcheck/256
Mailspring automatically checks your spelling as you type in the composer. Misspelled words will be underlined in red. You can right-click to choose from a list of suggested corrections or add the word to your dictionary.
For more information about how Mailspring implements spellcheck and how to adjust the spellcheck language, see this article.
For performance reasons, Mailspring will only mark only the first ten misspellings in each draft.