Cannot send e-mails

I have set up a btinternet  e-mail  account on Outlook using IMAP.  Everything works fine but every time I try to send a message "outlook cannot connect to server. verify that you are connected to the network' . The incoming IMAP server is mail.btinternet.com and the outgoing sever is imap.btinternet.com.  My settings for the  outgoing server is probably incorrect.  How do I correct this.