In order to find the Deletion settings on your server begin 
by launching the Microsoft Exchange System Manager.  Drill 
down to the Mailbox Store, (not the server icon).  Right 
click, properties, now select the Limits tab.  Half way 
down are the: Deletion settings.  Keep deleted items for 
(days), is the next dialog box.  See bottom of the diagram 
opposite.

Each Mailbox store has its own retention settings.  Your 
main decision is, 'How many days should we hold deleted 
mail on the server?'  I would have thought that 7 days 
would be about right.  Any longer and the mail will consume 
significant disk space on the server.  Any shorter and the 
users will feel frustrated because they cannot recover mail 
that they deleted only a few days ago.

if they have Deleted the mail from mail box like MS outlook
or outlook express and exchange server however we can
recover the mail from mail box. I know next question you
will ask me how here we have mail server is there can  we
recover the deleted mail from mail box and administrator can
set the setting in mail server or else user can set.if the
user delete the mail from mail box like if shift+delete even
we can't get from mail server also. if they delete like just
delete can recover the mail from mail box also as well as
mail server also..

It depends, if user has normal use del button to delete the mail and period is in the retention period so they can easly retrive the deleted mails with Deleted Items.

Hi, 
Yes, you can recover deleted mails from exchange server. But for that you have check your Deletion settings from Exchange System Manager. First goto Administrative Groups->First Adm. Group->Servers->'your server name'->First Storage Group->Mailbox store...
Now Right click on it and go to Properties->Limits 'tab'-> Deletion Settings, and make sure that your deleted mails are not longer that the number of days specified there.

Now to recover your mails, If you are using Outlook go to Tools->Recover Deleted Items, and you will be prompted a dialog box, using this box you can retrieve your desired mails. 
If you are using Outlook Web Access, Go to Deleted Items, in the tool bar you will find a button Recover Deleted Items, very next to Empty Delete Items. Here also you will find a window, using it you can recover your mails.

Hope this information will help you.