Windows 7: Outlook 2007 slow throughput for attachments

Ok so is no confusion the issue isnt a slow connection to the mail server or slow to submit email it is a low throughput. Since normal emails are tiny they go fast but if I eg. send a 1meg file over smtp email it is slower than eg. ftp.

My connection is 20mbit upload so approx 2.4mbytes upload so no confusion.

So my web server which I manage is my primary email server, it also has ftp as a transfer protocol. As well as a webmail interface.

If I upload the file to the ftp it uses all my upstream bandwidth.
If I send an email with the file as an attachment using webmail it uploads and is sent in under a second.
If I send the email via smtp using outlook it sends at around 200kB/sec.

The following all have no affect on the speed other than stated.

Disabling a/v
Changing the smtp port.
Disabling tls/ssl.
Enabling ctcp increases the speed to 250kB/sec, which is still hideous barely over 10% of my possible speed.

I havent tried another email client such as outlook express or thunderbird, or outlook 2010.

I suspect the issue is that outlook is hardcoded to a small tcp send buffer as a typical end user doesnt have a fast upload connection when outlook 2007 was developed. If so is there a way to tune it higher (or is it really hardcoded non tunable) or is there other ideas to speed it up.

Have you checked outlook for any possible addins? Your anti virus may have added a plugin.

As a test, open outlook in safe mode, (not windows safe mode )
Send a test and track its speed.

Your outgoing smtp is your broadband providers right ?

the anti virus does have an addin but its been disabled for testing.

the smtp server is my own smtp server I personally manage not my broadband isp.

Since I still havent got this resolved, I will try thunderbird as well as a newer outlook to see if they have the same issue. As I still believe its due to a hardcoded low send buffer value.