With NE, if you have a full backup prior to the time of the deletion (as bartounet says), then you can restore the account to a dummy account name and recover the email.
If you don't have a full backup prior to the time of the deletion, I think you still
might be able to scrounge some of the mail data from the redolog. In FOSS, you'd want to look in /opt/zimbra/redolog and (I believe) /opt/zimbra/redolog/archive; in NE, you'd also want to look at the incremental sessions in /opt/zimbra/backup/sessions. This will probably only work if the message was created or received during the interval represented by the redolog in question.
The redologs are somewhat readable in a text editor;
zmredodump might give more readable output but I haven't played around with it.