Windows Mail on Plusnet stops sending

[#1]

I went to see a client who is not particularly computer savvy and joined Plusnet on my recommendation.  All the emails she had sent in the last three weeks (about 80) were stuck in her Windows Mail (on Vista) Outbox.  There were two error messages, one suggesting the SMTP server authentication was wrong and the second saying an email was incorrectly addressed.  Indeed the oldest email in the Outbox was incorrectly addressed, something along the lines of "Fred Smith <fred@smith.co.uk;"  so the ">" was missing.  But SMTP was using the same credentials as POP3 and emails were still coming in.  The SMTP server, relay.plus.net was set up correctly.
I removed the offending incorrectly addressed email to the Drafts folder and all the others then proceeded to be sent as normal (so nothing was really wrong with the SMTP server authentication, which I didn't fiddle with).  Why did this "bad" email block all the others and is this a Plusnet issue or a Windows Mail Issue or a general email issue?     

[#2]

I would say Windows issue.

[#3]

Yes, but what I mean is how can I stop it happening again?  If it's a "Windows issue" that is intrinsic to the way Windows handles emails then there is no solution because switching to Linux or a Mac are not viable options, even if that would solve the problem.  If it's specific to Windows Mail then I can try another Windows-based email program as an alternative, Thunderbird maybe?

[#4]

I assume Windows Mail operates on a first sent/first transmitted basis with one item sent at a time. This will mean until that clears nothing else is sent as you observed. Here a malformed header is presumably tolerated by Windows Mail but rejected by the Plusnet relays. As far as the apparent authentication failure is concerned that malformed header could have upset processing further down the chain.
Lots of ifs and buts, however the relays seem to have made an effort to notify Windows Mail that all was not well but the latter failed to deal with the rejection. Whether a different email client would do better can really only be checked by experience.

[#5]

Thanks spraxyt, that all sounds very plausible.  So what I want is a more scrupulous email program that will check outgoing emails for malformed headers.

[#6]

Using Thunderbird 3.1.10 I tried entering "A Name" <somewhere.external.co.uk on the address line and Thurderbird refused to send it with the message shown in the attachment.
If I added the missing closing > it was sent (via the relays) and received at the external address.
So using Thunderbird looks to be a promising solution (at least for that problem).

[#7]

I've done the same test using Microsoft Outlook 2003 from a different computer. I typed the To address leaving off the closing > as before, However when I moved to the Subject line Outlook added the missing > (and removed the double quotes I'd put round the Friendly name). So it wouldn't let me enter an invalid To address.
Sending via the relays was OK and the message was received.