Personally I use webmail (RoundCube is my fave) almost exclusively. But if you are using an email program installed on your computer, you will need to set up the connection ports.
Below are the connection ports to use on Cpanel hosted accounts, please note that Plesk or Windows may be different:
993 (for SSL) (*Recommended)
143 (for non-SSL)
995 (for SSL) (*Recommended)
110 (for non-SSL)
465 (for SSL) (*Recommended)
25 (for non-SSL)
587 (for non-SSL)
* Recommended options provide the best security
Note: When using SSL connection (recommended) be sure to accept the server SSL certificate. That process is different for different mail programs.