What is Duolingo Czech's obsession with letters all about? We get so many sentences about letters and never one about emails or texts!
Well, e-mail is written the same in Czech, you don't have to learn it. We also say just mail/mejl, but that is colloquial. Text messages are called zprávy SMS, colloquially esemesky. It is a bit tricky question, whether to include the colloquial name.
It might be a good idea for tree 2.0 in the future to include these words.
I live in the Czech Republic so I do know these words. I think it would be more useful to learners of Czech in the 21st century to know mejl and esemeska, and that colloquial Czech should be included. Just my opinion.