iphone does not push subfolder - with email alias

hello zextras team, my customer complain that the iPhone does synchronize the sub folders
inbox
-- subfolder
-- subfolder
-- etc
- for your information it has one physical account and 4 alias emails.

1.it has one physical account > it works okay - because all these messages goes to inbox.
2. sub folder 1 > going here the first alias email - web access it is okay , but the iPhone does showing anything.
3. sub folder 2 > going here the second alias email - web access it is okay , but the iPhone does showing anything.

any ideas what happens ?

regards