When setting up POP3 account(s) in your e-mail software, you will need to enable SMTP authentication if you choose to use your hosting account for sending mail (instead of using your ISP's mail server).
The SMTP authorization login name / password is the same as your POP3 account login name / password.
How to do this varies by e-mail client. In MS Outlook: Tools, Accounts, select the account and click Properties. Then, under the Servers tab, check "My server requires authentication."
Please consult with your e-mail software's manual for further instruction.