outlook saying cannot connect to network whilst online

[#1]

I have set up my plusnet email account details ok in MS Outlook but when I test them or try sending e-mails I get the error message which says "unable to connect to the internet or network. Use the connections settings for more info" I have checked this and the connect via local lan is ticked.  I can get online etc OK but it will not send outgoing e-mail.  Would be gratefull for any suggestions.  I use a modem router and share a connection with another computer

[#2]

Do you use antivirus and/or firewall?  Which products?
What operating system do you use?
Do you get any error messages accompanying Outlook "will not send outgoing e-mail"?
Are you sending from the account mailbox or a personal one?  Have you entered the correct POP3 and SMTP server details, the correct mailbox username and password, ticked "Remember password" but *not* ticked "Log on using Secure Password Authentication"?

[#3]

Have you read Outlook Setup Guide?
http://www.plus.net/support/email/setup/outlook.shtml