Why my e-mail client deletes my emails from the mail server?

By default the protocols for incoming mail POP3 letters are copied from the e-mail client locally on the client computer, then automatically deleted from the server. This setting can be changed so that letters can be downloaded locally on the client computer and a copy of the letters is maintained on the server, ie letters duplicated, while maintaining both places. Below describes how becomes the setting for various e-mail clients.

MS Outlook and MS Outlook express

Menu Tools-> E-mail Acounts -> E-mail-> View or change existing e-mail
acounts -> Next -> Select question Account -> Change -> More Settings -> Advanced -> Place a checkbox option "Leave a copy of the massage on the server".
The settings are retained by click OK. It is desirable, e-mail client to reboot to take effect the new settings.

Mozilla Thunderbird

Menu Tools-> Acounts Settings -> From the menu choose "Server Setting" immediately questioned e-mail account -> From the right tick in the box to Leave messages on server.
The settings are retained by click OK. It is desirable e-mail client to reboot to take effect the new settings.

In an analogous manner to make this adjustment, and other e-mail clients, may be a difference in the menu names.
Another option for indelible letters from the server, reading them with mail client is the client be set to check mail via IMAP. IMAP e a quite new and better protocol than POP3. The advantages are that the letters do not need to be downloaded from the client to the user, and can be read and managed directly on the mail server.