System-wide configuration files are in /etc. The configurations there affect all users.
Each user has a home directory: /home/<username>. When you use an application, a hidden file is created in your user home directory which contains your user preferences for that application.
If you are using the Kde environment, open Konqueror (the file manager). Open your home directory. Click on the View menu, then select "show hidden files". The hidden directories/files have a dot as the first character in the file name.
The config file you need to edit is probably somewhere in /etc.
From a console window (the command-line window), switch user (su) to root. That means enter the command su, then enter the root user password at the prompt.
Then run this command: find /etc -name '*.*' -type f | xargs grep 'user@mydomain.com'. That should tell the the name and path to every file in /etc which contains the string
user@mydomain.com.