This study focused on examining the overall effect of using email on the improvement of writing performance in aspects of syntactic complexity, grammatical accuracy and lexical density, as well as investigating the relation between the number of email exchanges and writing performance. Subjects in this study were 40 non-traditional EFL students enrolled in an intermediate reading class at a university in Taiwan. By applying qualitative and quantitative research methods, the major finding demonstrates that students made improvements on syntactic complexity and grammatical accuracy. An increase in lexical density, however, was not observed in this study. Another finding suggests that exchanging email messages with their peers at least four times might have a greater overall improvement on their writing performance. Findings from students' self-reports reveal that the email approach was a positive strategy that helped improve students' foreign language learning and attitudes towards English. Instructional implications for designing an effective email task to enhance foreign language writing development and attitudes are also presented. (Contains 4 tables.)