Large emails hang up Outlook

[Ralph Page]

My webhost recently changed email servers and since that time I've
noted that emails larger than roughly 100K seldom get through.

I used a packet sniffer to examine the exchange and it appears to be
identical to that for smaller emails that work.
When the final packet from the server arrives, I (Outlook) ACKs it
with the expected sequence and acknowledgement values but it does
nothing further. After 5 minutes the server times out.

After several (up to 20-30) tries spanning a few hours to a few days,
the large email typically gets through and the exchange looks
identical except that Outlook responds with the expected request to
retrieve the next email rather than just sitting quietly after the
final packet is received.

How does Outlook determine when the data has been completely sent?

I noted that the server provides the size in octets and I also noticed
that the final packet from the email server always has the push (PSH)
flag set in the TCP portion of the header. The final ACK from me
contains the expected sequence and acknowledgement values.

Thanks for any light anyone can shed on this, the webhost has been
less than helpful in helping to track this down.

-Ralph Page

 

[Brian Tillman]

I used a packet sniffer to examine the exchange and it appears to be
identical to that for smaller emails that work.
When the final packet from the server arrives, I (Outlook) ACKs it
with the expected sequence and acknowledgement values but it does
nothing further. After 5 minutes the server times out.

After several (up to 20-30) tries spanning a few hours to a few days,
the large email typically gets through and the exchange looks
identical except that Outlook responds with the expected request to
retrieve the next email rather than just sitting quietly after the
final packet is received.

This sounds like what would happen if you employ a mail scanning app, like
an antivirus or -spam program. Programs which scan incoming mail interfere
with the proper interchange between the mail client and server.

How does Outlook determine when the data has been completely sent?

If you're looking at the SMTP connection, then Outlook DOESN"T determine
when the data has been sent. The server determines when it has been
received and tells Outlook so.

I noted that the server provides the size in octets and I also noticed
that the final packet from the email server always has the push (PSH)
flag set in the TCP portion of the header. The final ACK from me
contains the expected sequence and acknowledgement values.

This sounds like the POP connection, which is different than the SMTP
connection and doesn't contribute to whether or not a message gets sent.

Thanks for any light anyone can shed on this, the webhost has been
less than helpful in helping to track this down.

Rather than using a packet sniffer, use diagnostic logging in Outlook to see
if the interchange is completing properly and disable any mail scanning
applications you have.
http://support.microsoft.com/?id=300479