As a preliminary study for the effective development of a genre-focused English learner corpus, this article aims to investigate most frequent error types and their frequencies in English emails written by undergraduate freshmen in South Korea. Data for this study include English emails of 86 Korean students majoring in humanities or social science in a university located in Daegu. With the rise of Internet, ESL/EFL education has witnessed a growing interest in teaching email usage in composition courses, as it provides a variety of opportunities to evaluate language abilities including interpersonal and pragmatic abilities. The present article revealed that the most frequent error type was concerned with style, such as capitalization (28.7%) and punctuation (7.8%), which was followed by determiner deletion (6.3%), genre convention such as closing (3%), countability of nouns (3%), and verb choice (2.7%). Different error types and frequencies were identified according to different English proficiency levels (Korean SAT and TOEIC), which evidenced the need to include the English proficiency level annotation in the corpus design and to focus on different types of errors in class in accordance with learners’ proficiency levels.