Why claws mail can receive email instead of mail command?

Question

mail -f /var/mail/debian9
/var/mail/debian9: 0 messages

When mail -f /var/mail/debian9 executing ,the claws mail application is in off status.

Now open the /var/mail/debian9 with claws mail.

sudo apt-get install -y claws-mail

claws mail is here claws mail

Claws-mail say there 665 total 655 unread.
Why mail -f /var/mail/debian9 display none of email?

Answer 1

claws-mail will create a directory ,for example in my pc, /home/debian9/Mail.

ls -al  /home/debian9/Mail 
total 40
drwx------  7 debian9 debian9  4096 Apr 25 17:50 .
drwxr-xr-x 65 debian9 debian9  4096 Apr 25 17:59 ..
drwx------  2 debian9 debian9  4096 Apr 24 11:08 draft
drwx------  2 debian9 debian9 16384 Apr 25 17:39 inbox
-rw-r--r--  1 debian9 debian9     0 Apr 25 17:50 .mh_sequences
drwx------  2 debian9 debian9  4096 Apr 24 11:08 queue
drwx------  2 debian9 debian9  4096 Apr 24 10:47 sent
drwx------  2 debian9 debian9  4096 Apr 24 11:45 trash

claws-mail find some email in my /var/mail and /var/spool/mail and change the format ,saved them in .mh_sequences. The displayed email are all in .mh_sequences.