Actually servers as illustrated in the above picture some times leak email ids which help spammers to pin point an email address to spam ! As seen above xt3m3@<target host.com> didnt work because it didnt exist and the server gave a message user unknown ! Attempts are made until a legit email isnt discovered thus the server giving a reply Recipent Ok .
This not a serious threat but then again once emails are attained then sending trojans , worms and virus is a normal practice ! But mostly it is used for Spamming !
Cyber Security Blog 