Lexical bundles are worthy of attention in both teaching and testing writing as they function as basic building blocks of discourse. This corpus-based study focuses on the rated writing responses to the email tasks in the Canadian English Language Proficiency Index Program® General test (CELPIP-General) and explores the extent to which lexical bundles could help characterize the written responses of the test-takers of different English proficiency levels. Three subcorpora of email writing responses were created based on test-takers' proficiency levels. AntConc 3.4.4 was used to identify 2- to 6-word lexical bundles, which were then manually coded for their discourse functions. The results showed that test-takers of higher proficiency levels used more lexical bundles in terms of both bundle types and tokens compared with test-takers of lower proficiency level. The writing samples of different proficiency levels shared some lexical bundles, and overall they had similar proportional distributions of lexical bundle functions. Nevertheless, noticeable differences among the proficiency levels were observed in the proportional distributions of the subfunctions of stance bundles, discourse organizing bundles, and bundles of other functions. The identification of differential use of lexical bundles can contribute to a better understanding of English learners' email writing performance.