What provider are you with, as I know eircom don't allow other providers ip's to send mail through their mail servers.
You have 2 work arounds - use their webmail, which can be used to send/receive from anywhere - this would replace you pop application (outlook/outlook express)
Other option is they have pop before send authentication, not sure if this is allowed for free email acconts but this temporarily allows users from other providers to send mail - if you check for mail first (not send/receive) then try sending.