Once every three days（三天一次）
Email is done online
I’m mailing you a letter tomorrow
Letters is sent by “postal system”(邮政系统)
Mail(Letter) : is a sent letter or (package) using postal system
Email is used more than letter
Formal (正式的)letters are the hardest，Because you have to make sure your grammar is flawless( /ˈflɔːləs/)
job application are the hardest(adj.)
Flawless =perfect they are synonym
I don't know what the Email said
so I have some nervous
Your feelings change depending on who sent the email
When I receive an admission or invitation email I feel excited. (Overjoy-/,əʊvə'dʒɒɪ/ v. 使狂喜，使万分高兴)
I often communicate with coworkers using email if the work is urgent as it is more convenient than texting.
Texting is sending messages .
Chat is more of like speaking