Despite the inability to change the username on the mail service, the solution to this problem is still there. Get a new email address and configure it correctly. The described method refers to a system of Yandex, on other mail services operate the same way. The headings or buttons may be of different names, but they are the same.
Open the page and click to "Get mail" or open the front page and Yandex click on the line link "to Have a mailbox" in the left part of the window under the login form in the mail.
Complete the two step registration. You will need to choose a new username, create a password and provide personal information. Better specify valid information, the system will be easier to recover your password if you forget it. After registration log in to your new mailbox.
In the upper right corner of the window immediately under your email address click on the line link configure. You can also click on the link "Customize" in the left part of the screen (under the list of folders) and after the transition to the new page to climb to a higher level.
On the settings page, click the "Collect mail". Yandex collects mail from any servers that support the POP3 Protocol. In the field E-mail specify the address of the old post box, in the field "Password", respectively, enter the password for it.
Decide whether you need, to mail collection letters were left in the old mailbox. Depending on the selection, select or clear the marker from the field "to Keep the original letters in the box". Click on the "Enable collector". Wait until the connection is established, specify in which folder to put the collected correspondence. Click on the "Save changes".
If the connection fails, click on the settings page, the row-link "mailers" and make sure that the fields "From the server according to the POP3 Protocol" and "server IMAP" are markers. Save your changes and repeat the steps above. Then send the new box, and letters at your old address will be redirected to your new mail.