Problem displaying old messages

Hi, I'm testing zextras mobile.
I noticed that if I set for example one month as synchronization interval in account settings on mobile phone, I can't see messages older than one month if I scroll down the email list.
If I make a search in emails, the phone presents me only results from one month; at the end of the list there is a link "load more", but if click on it a circle loanding appears but then no other message is loaded.
I tested on different phones with same result.
Is it a normal behaviour? With z-push this does not happens.
Thank you very much.