If you send files regularly with confidential information you may want to use a secure mail server. Therefore I let you this list with some of the best email servers. I do not pretend that are all but I hope these will be useful to you. If you know some other comments and I'll add to this list.
ShazzleMail
Hushmail.com
StartMail.com
TorGuard.net
RiseUp.net
OpaqueMail.org
Autistici/Inventati
NeoMailBox.com
4SecureMail.com
CounterMail.cm
CryptoHeaven.com
S-Mail.com
Securenym.net
Safe-Mail.net
KeptPrivate.com
Novo-Ordo.com
LockBin.com
AES.io
Opolis.eu
OneShar.es...
