So it is safer not to have it in mailbox then to have it?
There are two basic levels of protection when it comes to your wallet.dat file:
(1) Don't let people get hold of a copy.
(2) Keep it encrypted (an option in most clients) with a very strong password. If someone does get hold of a copy, they might not be able to crack the password.
You are making (1) more difficult for yourself.
Obviously the more copies you make and the more places you send them, the more likely it is someone else will get hold of a copy.
If someone were to gain access to your email, for example, Bitcoin related messages might prompt them to do a quick search for a wallet back up.