The E-mail sender's address book is safeguarded from computer virus intrusion with only minimal interference with normal E-mail distribution from the sender. Complete encryption of the address book create interference with the send function in that before any E-mail function may be started, the address book must be accessed and decrypted even when the E-mail being sent does involve the address book. However, the address book may be safeguarded from the computer virus with minimum E-mail interference if only a part of the address book is encrypted. There is stored the unencrypted data representative of a plurality of aliases for plurality of addressees of electronic mail, and separately stored encrypted data representative of the electronic mail addresses of said plurality of addressees. In response to a request to send electronic mail to one of said aliases, the entry of the key to decrypt the encrypted data is required to thereby provide the address of the addressee having said alias. If no alias is entered, i.e. the full address is entered, then the E-mail is sent without requiring a key entry by the sender.