MODIFY CFDEF_STANDARD.PHP
Find this piece of code:
-----------------------------------------------------

$g_custom_field_type_definition[ CUSTOM_FIELD_TYPE_EMAIL ] = array (
	'#display_possible_values' => TRUE,
	'#display_valid_regexp' => TRUE,
	'#display_length_min' => TRUE,
	'#display_length_max' => TRUE,
	'#display_default_value' => TRUE,
	'#function_return_distinct_values' => null,
	'#function_value_to_database' => null,
	'#function_database_to_value' => null,
	'#function_print_input' => 'cfdef_input_textbox',
	'#function_string_value' => 'cfdef_prepare_email_value',
	'#function_string_value_for_email' => 'cfdef_prepare_email_value_for_email',
);

-----------------------------------------------------
and change this line:
-----------------------------------------------------
	'#function_print_input' => 'cfdef_input_textbox',

-----------------------------------------------------
to this:
-----------------------------------------------------	
	'#function_print_input' => 'cfdef_input_email',


-----------------------------------------------------
in the same CFDEF_STANDARD.PHP, add these code at the end:
-----------------------------------------------------

function cfdef_input_email($p_field_def, $t_custom_field_value) {
    # By XFMM
    # Split the value into address and domain
    $t_domains_list = custom_field_prepare_possible_values( $p_field_def['possible_values'] );
    $t_address_part = $t_custom_field_value ;
    $t_domain_part = '' ;
    if( $t_domains_list <> '' ){
        $t_position_at = strpos($t_custom_field_value,'@');
        if( $t_position_at > 0 ){
            $t_address_part = substr($t_custom_field_value,0,$t_position_at);
            $t_domain_part = trim(substr($t_custom_field_value,$t_position_at+1));
        }
    }
    # Let's begin with a single input field.
	echo '<input ', helper_get_tab_index(), ' type="text" name="custom_field_' . $p_field_def['id'] . '" size="40"';
	if( 0 < $p_field_def['length_max'] ) {
		echo ' maxlength="' . $p_field_def['length_max'] . '"';
	} else {
		echo ' maxlength="255"';
	}
	echo ' value="' . $t_address_part .'"';
    echo '></input>';
    # Now, let's add a list with commonly used domains, taken from the "possible values" argument.
    $t_domains_list = custom_field_prepare_possible_values( $p_field_def['possible_values'] );
    if( $t_domains_list <> '' ){
        echo '    @    ';
	    $t_values = explode( '|', $t_domains_list);
	    $t_list_size = 0;	# Treat this as an ENUM, not a list.
	    echo '<select ', helper_get_tab_index(), ' name="custom_field_' . $p_field_def['id'] . '_domain" size="' . $t_list_size . '" style="width: 220px" >';
	    $t_selected_values =  $t_domain_part ;
        $t_selected_value_found = false;
	    # Start with an empty entry.
	    if( $t_domain_part == '' ) {
	        echo '<option value= "" ></option> selected="selected"> ';
	        $t_selected_value_found = true;
         } else {
	        echo '<option value= "" ></option> ';
         }
	    foreach( $t_values as $t_option ) {
            if( $t_option == $t_domain_part ) {
			    echo '<option value="' . $t_option . '" selected="selected"> ' . $t_option . '</option>';
			    $t_selected_value_found = true;
		    }  else {
			   echo '<option value="' . $t_option . '">' . $t_option . '</option>';
		    }
	     } #Foreach
	     # It�s possible to have a value not included in the predefined list.
	     if( !$t_selected_value_found ) {
		     echo '<option value="' . $t_domain_part . '" selected="selected"> ' . $t_domain_part . '</option>';
         }
	     echo '</select>';
     }
}

-----------------------------------------------------
Now, modify GPC_API.PHP 
Find this function:
-----------------------------------------------------

function gpc_get_custom_field( $p_var_name, $p_custom_field_type, $p_default = null ) {

-----------------------------------------------------
And just before the 'default' Statement of the 'switch' structure, add the following:
-----------------------------------------------------
	case CUSTOM_FIELD_TYPE_EMAIL:
             # XFMM: Mix address + domain.
             $t_value = gpc_get_string( $p_var_name, $p_default );
             if( strpos($t_value,'@') == 0 && $t_value<>''){
                 $t_var_name_domain = $p_var_name.'_domain';
                 $t_value .= '@' . gpc_get_string ($t_var_name_domain,'') ;
             }
             return $t_value;
             break;


-------------------------------------------------
Finally, go to manage your custom field, and add some domains in the "possible values" field, to see your solution working!.
-------------------------------------------------