Don’t know why, the usermin only shows four days emails (~10 emails). However when I go to file manager, I found there are a lot of email. And also, the size is several GB. How to solve such issue?
Kindly specify which mailbox in Usermin are you looking at and which directory in the file manager leads you to believe that there are mail from several days amounting to several GB there.
The mail box:
Based on the answer you have provided I suspect you are not looking at the mailbox which shows the mail in the directory containing several GB of mail. The mailbox you are looking at is associated with a different directory. The directory containing several GB of messages is associated with a different mailbox.