Why can't I send or reply to emails from webmail?

[#1]

We've had this problem for a while but usually use Outlook for emails.  However we're away at the moment and can't send or reply to emails on webmail, always comes up with an error message, something about authentication.  Can anyone help please?  I am fed up of copying the addresses over to Hotmail to send a message!  Thank you.

[#2]

How are you connecting to Webmail? On someone else's PC via a broadband connection or some other way?
Is the connection secure (always showing https)? Are you logging in here https://webmail.plus.net/ and now using Roundcube or still logging into Squirrelmail?

[#3]

On someone else's computer but it is secure and showing https.  And using the new version I think, its definitely changed since I last used it.

[#4]

Hmm. It may not be liking things because it's possibly not on a Plusnet connection and as I haven't logged into Roundcube on an non-PN connection yet, I can't be more specific, but it should be possible to authenticate. Hopefully someone using non-PN connections regularly maybe able to help.
(I'm assuming you are logging in via a Browser and not a Client - if the latter, check the server settings).

[#5]

I assume you can log into webmail so it's just sending that doesn't work.
Do you have the correct default identity (Display name and Email address) set up on the Personal Settings -> Identities tab? (No accountname+mailboxname in this address.)