[Evolution] trouble replying to emails

[Dave Howorth <dave howorth org uk>]

I use Evolution on my Linux box and have been using it successfully for
years. I'm in the UK and I've just changed my ISP from Eclipse to
Plus.net and have encountered a problem with replying to messages.

I can receive messages OK, and I can write new messages (like this one)
and it will be sent OK, but if I try to reply to an incoming message, my
reply just sits in the Outbox. If I copy the subject, text and
addressees to a new message, that will be sent OK.

Sometimes I see a dialog box with the message:

  Error while Sending message.

  Welcome response error: smtpout.karoo.kcom.com

  Welcome response error: smtpout.karoo.kcom.com

I googled the error message without much success, except to see a
Thunderbird user with an unsolved similar problem. I started Evolution
with debug turned on and this is what I see in the log:

Thread 0 >
CamelFolder:get_message('Outbox', '1686') =
class: CamelMimeMessage
mime-type: text/plain; charset="UTF-8"
content class: CamelDataWrapper
content mime-type: text/plain; charset="UTF-8"
< 0 >
CamelException.setv(0x7f54e3866ba0, 2, 'Welcome response error:
smtpout.karoo.kcom.com')
Thread 0 >
CamelFolder:get_message('Outbox', '1688') =
class: CamelMimeMessage
mime-type: text/plain; charset="UTF-8"
content class: CamelDataWrapper
content mime-type: text/plain; charset="UTF-8"
< 0 >
CamelException.setv(0x7f54e3866ba0, 2, 'Welcome response error:
smtpout.karoo.kcom.com')
CamelException.setv(0x1d86a20, 2, 'Welcome response error:
smtpout.karoo.kcom.com

(there are two messages in the Outbox)

The (munged) headers from one of the messages are:

Subject: Re: Fwd:
From: me <me example com>
To: addressee <addressee gmail com>
In-Reply-To: <long-id gmail com>
References:
         <long-id.some-domain.com>
         <long-id gmail com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Evolution-Format: text/plain
X-Evolution-Account: 1167672590 8188 0 piglet
X-Evolution-Transport:
 smtp://me%40example eclipse co uk smtp eclipse co uk/;use_ssl=never
X-Evolution-Fcc: mbox:/home/me/.evolution/mail/local#Sent
X-Mailer: Evolution 2.30.1.2 
Date: Sat, 11 Aug 2012 23:00:47 +0100
Message-ID: <1344722447 6342 5 camel piglet example com>
Mime-Version: 1.0

One thing I don't understand is why there is any mention of eclipse in
the headers? I've set Plus.net as my default account.
I'd appreciate any thoughts on what the problem is or how to diagnose it
better. Is this my finger trouble in settings, or an Evo problem, or a
Plus.net problem?

Thanks, Dave