Mail cannot send attachments

[#1]

MBP Mac OS 10.6.8
Mail Version 4.5 (1084)
GMAIL
Gmail can send attachments OK
Mail can send email OK
Mail cannot send email with attachments
Cannot send message using the server smtp.gmail.com.xxxxxxxx@gmail.com
etc is the message header that comes up
Mail Connection Doctor details that connection and login to server succeeded for both Gmail and smtp.gmail.com.xxxxxx.
I've tried trashing various plists and reinstalling the Mail app. All to no avail.
Is this a Plusnet ISP problem?
Is Plusnet blocking the sending of attachments?

[#2]

How big is your attachment that you are trying to send ?

[#3]

from 8kb to 1.4Mb
Mail seems to be rejecting all attachments

[#4]

The maximum total size of an email accepted by Plusnet is 100MB.

[#5]

As it's GMail, what type of attachment are you trying to send?
They will not allow you to send a significant number of file types, including executables, even if the are in disguise.
As has been indicated, what happens if you use relay.plus.net as your outbound server and not the GMail one?
Note: you will need to disable outbound authentication whilst doing this, to ensure you are not rejected due to incorrect details.

[#6]

I added relay.plus.net as my outbound server and yes I can now send attachments.
However the outgoing message speed is very sluggish (with attachments)
I  have also changed the DNS to 8.8.8.8; 8.8.4.4 to see if that speeded things up.
I am now getting athentication issues.
This message may not have been sent by xxxx@gmail.com
thanks

[#7]

Changing DNS will not speed up any transfer speeds, only initial connections and generally only the first connection within a period of time.
The authentication error is due to the way that some email providers including AOL and GMail setup their email domain (the bit after the @ symbol) to give additional authentication.
If you are interested in the technical details, see the following article - http://en.wikipedia.org/wiki/DomainKeys
The idea of that authentication is that it shows it was sent by the sender. For GMail, that is done by using their web interface or using their SMTP server.
Back to the slow speeds through. What is the file size of the attachment and just how long does it take to send?