Do you really use both of them together all the time in your emails, or are you just torn between the two? Well, the difference is that "therefore" marks a **logical consequence** (you do something and a result necessarily follows from it), whereas "then" is more **temporal or circumstantial** (it happens at the same time or it's a reaction to what was just said). For example: "I forgot my file, therefore I need to get it" (direct consequence), versus "We were discussing the project, then Marie had a good idea" (it happened during). In a professional email, "therefore" is safer because it sounds more formally logical, whereas "then" can sometimes seem a bit too conversational. Personally, I always reread my stuff by asking myself if I could swap the word for "as a result" - if it works, that's a good sign that it was "therefore" you needed!