Go to your Outbox folder, make sure that View, Current View, Show all
Messages is selected.  Delete any messages there.  Then try again.  If
you had a bad message there, OE will try to resend it every time you do
a send until it either works or you manually delete the message.

If you still have problems, make sure that your anti-virus program is
NOT set to scan outgoing e-mail.  With some anti-virus (and anti-spam)
programs, simply disabling the e-mail scanning function doesn't really
disable it.  It simply tells the program not to flag any messages.  It
still processes all the messages and may still cause send/receive
failures.  What you may need to do is uninstall the anti-virus program,
then reinstall it using a custom install option and unselect the e-mail
scanning component for installation.

And if you still have problems you'll need to contact btinternet.com.

I've never known OE to have a problem generating bare linefeeds as part
of the message, so I don't think it's an OE problem.  Although if you
were sending a text file as an attachment and it had bare linefeeds, OE
might use quoted-printable encoding which might cause the problem for
the SMTP mail server.  In that case, zip the text file before sending
and the problem won't occur.

A couple of things to try:

- Enable SMTP authentication in the e-mail account defined in OE.

- Disable e-mail scanning by your anti-virus program.

Did you read the link? => http://pobox.com/~djb/docs/smtplf.html  ...does
any of it make sense to you?

Is your anti-virus application configured to scan outgoing (and incoming)
mail?  Is this a sudden, new problem?