Recently one of my clients received the following error in Windows Mail on Windows Vista:

Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Subject ‘Fw: ‘, Account: ‘BT Yahoo! Mail’, Server: ‘’, Protocol: SMTP, Server Response: ‘451 See’, Port: 25, Secure(SSL): No, Server Error: 451, Error Number: 0x800CCC0F

I discovered that the error was described as

Bare LFs in SMTP. This error means very little to most people.

I found that once the email was deleted from the outbox the error would go away.

However, later the error returned.

I found that the user was selecting “Forward as an Attachment” instead of simply “Forward”.

It seems that the mail server (probably qmail) supplied by the ISP did not like the email (with attachments) being forwarded as an attachment.

The solution is simple.

Don’t use “Forward as an Attachment”, instead simply use “Forward” which does almost the same thing. Of course you also need to delete that bad email from your outbox too before the error will permanently disappear.

A simple error, but unless you know what you’re looking for, you probably won’t realise what’s going on.

I hope this helps.