Unable to send "large" (>500kB) mail attachments - internet connection lost?

[david.f.jenkins]

I have two PCs behind a router which is connected to an ISP via
broadband wireless. The PCs are connected via ethernet to the router.
On one PC ("PC1"), I have no problem sending "larger" files as
attachments to e-mails. These e-mails can be sent from Outlook, or
from the mail server's (and I've tried two different outgoing mail
servers) web interface - it matters not. On the other ("PC2"), I am
only about 5% successful in sending these larger files. The larger the
file (I have tested up to 11MB), the greater the incidence of problems.
(The attachments, BTW, are always PowerPoint files.)

Here's what happens on PC2: when the attachment is being uploaded,
anywhere from 1 second to 30 after the upload starts, it appears that
the Internet connection is lost. That is, if I'm using the web
interface (instead of Outlook), after a short time, I'll be reverted to
a "Page Not Found" window, and the Send is aborted. If I'm sending
from OUtlook, the Send ends with an 0x800ccc0f error.

The TCP/IP settings of PC1 and PC2 are identical. The NIC drivers on
PC2 are updated to the latest available from the MFR (RealTek). The
Outlook e-mail accounts are configured identically on both PC1 and PC2.
I have McAfee VirusScan disabled for both incoming and outgoing
e-mail. (As a matter of fact, I've unstalled McAfee and tested, and
still got the error.) I've teste with Windows firewall ON and OFF.
I've added Outlook.exe to the Windows firewall exception list, but
nothing changed.

I think I've eliminated AV programs and Outlook as sources of the
problem, so I'm thinking some sort of well-hidden network problem(s),
and need some expert help diagnosing. I've also got the ISP's ear on
this (he's a neighbor, and interested), so I feel confident that I can
get them to gather any diagnostic info that might be required.

Can you suggest new diagnostic info that might help me solve this
problem - it's driving me crazy!

 

[david.f.jenkins]

I'm rethinking my statement that Outlook is a not source of the problem
- when sending an attachment using a mail server's web-based interface,
does Oultook Express get involved? I ask that, because FTP always
works - it's only e-mail (from Outlook, or via web interface) that
fail.

Just wondering...

 

[david.f.jenkins]

And here's even more input: If I look in IE Tools -> IE Options ->
Programs I see that Oultook is my default e-mail program. What does
that really mean? I normally get into Outlook by clicking on its icon.
Where else might it get invoked? More importantly (I think) when I
send mail using a mail server's web interface, does Outlook get
involved? Outlook Express? And how do I tell which, if so?

 

[david.f.jenkins]

Just to wrap this up and close it out: I lied in my first post - the
Tcp setting were _not_ the same between PC1 and PC2 - in particular,
SackOpts = 1 was set in the "good" pc, and the bad PC had TcpMax
DataTransmissions set to 1, where on the "good" pc it had been set to
(default).

With those changes (and perhaps, under goading, my ISP tweaked his
radio), I've been able to send these files without problem for the last
72 hours.