Typically the changes that the end user needs to make are minimal. They will need to change the username on the email client they use to POP the correct account.
For example, if the user is using Outlook, select Tools, Accounts and then select the Mail tab. Highlight the mail account that is affected by the change in the list and then click the Properties button.
Once into the account properties, select the Servers tab.
Change the Account name to the name of the user account that was created in the Secondary domain ( seconddomain.com\webmaster ) and then click Apply to make the change.
Note: The Account name must be entered as Domain Name\Username.