this answer is only applicable for email accounts hosted by us.

let us assume that you have this email address:
fred.flintstone@theQuarry.com
with a password of: rockstone

MAIL
the POP (incoming mail) settings are:
pop: mail.theQuarry.com
username: fred.flintstone@theQuarry.com
password: rockstone

the SMTP (outgoing mail) settings are:
smtp: mail.theQuarry.com
username: fred.flintstone@theQuarry.com
password: rockstone

the SMTP MUST be set to Authenticate

if you are using Outlook Express
go to Tools > Accounts
then select the appropriate account (I assume that you have several)
click "Properties" then the "Servers" tab
at the bottom, tick the box "My server requires authentication" then
click the adjacent "Setting" button, click "log on using"
and enter the smtp details

...return