[LAB2] 312551178

[HanHongChen]

Description

完成MailSystem、Application test

• A clear title (name your PR "[LAB{lab_number}] {your_student_id}")
• A meaningful message for PR, as well as its commits
• From your specific branch (not main or other's branch) merging to your branch
• Excluding any irrelevant files, such as binaries, text files, or dot files
• Passing all CI (You should check it first to pass one of the validations in CI. However, you need to make sure your PR passes all CI after you submit it.)