Initially I created a user account named 'user3' later on I deleted simply using userdel command. When I again tried to make a user account with same name as user3, following message appeared:

Creating mailbox file: File exists

I came to know that best way to delete the user account is userdel -r user_name. Now I get above message every time when I try to create account named "user3".

Any suggestion, How can I get rid of these problem???

I solved the problem.....

please tell us how.
posting on a forum always goes both ways - others might benefit from your solution.
and please mark the thread as solved.

"Creating mailbox file: File exists" is more a warning - the easy fix is just to delete the file.

To fully clean out a user requires "userdel -r <username>". This deletes the users home directory (and all the file), and deletes the users mail file. Without the -r, it only removes the login name and entries from the groups file. Frequently, this is preferred because somebody usually has to go through the files and decide what to keep, or to discard. That includes the mail file...

So leaving these around generates warnings about things that exist, that possibly shouldn't.