Apparent MTU Problem

I'll try to make this short and sweet.

I have a Windows Server 2003 Web Edition computer that I am running Ipswitch
IMail Server 8.15 on. Our dial-up customers are located in a colo facility
in another city. Until recently, this e-mail server was located in my
office and customers had to go out over the public internet to retrieve
their e-mail. A few weeks ago, I moved the e-mail server to the colo
facility, so now the access servers and e-mail server are on the same
Ethernet switch.

I have had no problems for 3 1/2 years using the e-mail server in the former
setup. However, now that the dial-up customers are on the same Ethernet
switch as the e-mail server, the e-mail is much slower and customers who are
trying to download large attachments cannot do so.

On the client side, Outlook Express downloads all messages up to the large
one, and then you see dial-up connection status lights in the system tray
quit blinking. Eventually Outlook Express pops up a box and says that the
POP3 server has been idle for 60 seconds, and asks if you want to wait or
stop. Clicking Wait does no good, it will never download past that point.

On the server side, using Ethereal (a packet logger), I have noticed several
things. First off, things obviously work smoothly until OE attempts to
retrieve the large message. When OE sends the command to retrieve the large
e-mail, things rapidly deteriorate. I start seeing alot of duplicate ACKs
coming from the client side. Some packets are never ACKed, and some ACKS
come up to 5-10 seconds, and out of order. All this time, though, the
server is still attempting to send packets to the client, but as I said, the
lights aren't even blinking on the client side.

The packets the server is sending are 1514 bytes. (Ethereal says 1514 bytes
captured off the wire, but in the IP section it says the header is 20 bytes
and the total length is 1500 bytes.) Everything doesn't add up, but
whatever, I'm no whiz, I'm just reporting what I found. My dial-up
customers' MTUs are set to 1500 bytes. I am able to ping the mail server
from a client machine (ping mail.ziggycom.net -f -l 1472) with no problem.
I started experimenting with the MTU in the registry, restarting each time
and trying to download mail to my laptop. Finally, at a setting of 1458 on
the NIC card, I was able to download large attachments to my laptop. (1478
+ 28 = 1472, see above. ??? I just know it worked.) However, this has not
fixed the problem for everybody and a lot of people are still having
problems.

HELP!

Thanks,
Bryan E. Martin
Ziggycom Online, Inc.

If no one can help me with this problem, could someone suggest a more
appropriate newsgroup?

Thanks,
Bryan

"Bryan E. Martin" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I'll try to make this short and sweet.
>
> I have a Windows Server 2003 Web Edition computer that I am running
> Ipswitch IMail Server 8.15 on. Our dial-up customers are located in a
> colo facility in another city. Until recently, this e-mail server was
> located in my office and customers had to go out over the public internet
> to retrieve their e-mail. A few weeks ago, I moved the e-mail server to
> the colo facility, so now the access servers and e-mail server are on the
> same Ethernet switch.
>
> I have had no problems for 3 1/2 years using the e-mail server in the
> former setup. However, now that the dial-up customers are on the same
> Ethernet switch as the e-mail server, the e-mail is much slower and
> customers who are trying to download large attachments cannot do so.
>
> On the client side, Outlook Express downloads all messages up to the large
> one, and then you see dial-up connection status lights in the system tray
> quit blinking. Eventually Outlook Express pops up a box and says that the
> POP3 server has been idle for 60 seconds, and asks if you want to wait or
> stop. Clicking Wait does no good, it will never download past that point.
>
> On the server side, using Ethereal (a packet logger), I have noticed
> several things. First off, things obviously work smoothly until OE
> attempts to retrieve the large message. When OE sends the command to
> retrieve the large e-mail, things rapidly deteriorate. I start seeing
> alot of duplicate ACKs coming from the client side. Some packets are
> never ACKed, and some ACKS come up to 5-10 seconds, and out of order. All
> this time, though, the server is still attempting to send packets to the
> client, but as I said, the lights aren't even blinking on the client side.
>
> The packets the server is sending are 1514 bytes. (Ethereal says 1514
> bytes captured off the wire, but in the IP section it says the header is
> 20 bytes and the total length is 1500 bytes.) Everything doesn't add up,
> but whatever, I'm no whiz, I'm just reporting what I found. My dial-up
> customers' MTUs are set to 1500 bytes. I am able to ping the mail server
> from a client machine (ping mail.ziggycom.net -f -l 1472) with no problem.
> I started experimenting with the MTU in the registry, restarting each time
> and trying to download mail to my laptop. Finally, at a setting of 1458
> on the NIC card, I was able to download large attachments to my laptop.
> (1478 + 28 = 1472, see above. ??? I just know it worked.) However, this
> has not fixed the problem for everybody and a lot of people are still
> having problems.
>
> HELP!
>
> Thanks,
> Bryan E. Martin
> Ziggycom Online, Inc.
>