Re: Attachment Limits

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From: Rich Matheisen [MVP] (richnews_at_rmcons.com.NOSPAM.COM)
Date: 01/02/05 

Date: Sun, 02 Jan 2005 14:17:19 -0500

Rajesh <Rajesh@discussions.microsoft.com> wrote:

>We have an Exchange 2003 server, which is accssed Via POP3/SMTP (OL Express &
>OL) over the internet. The Global Settings has a limt of 4 MB on the Message
>size. However when users submit messages with say 10 MB, the server does not
>give an NDR straight away. It looks like the message is attempted to be
>uploaded to the server before it can determine the size.

Whether Exchange (or any SMTP server) can determine the message size
before receiving the whole thing (or at least a goodly portion of it)
depends on several things:

1) whether the server advertises the possibility of using ESMTP
2) whether the SIZE keyword is advertised
3) whether the client understands ESMTP
4) whether the client adds the "SIZE=" to the MAIL FROM command

See RFC1870 and RFC1869 for the partculars.

If the SMTP client (Outlook Express, for example) tries to use the
EHLO command to initiate the SMTP session, but receives "command
unknown" or "command not implemented" it will not use ESMTP and
therefore cannot use the SIZE= parameter on the MAIL FROM command.
This means the entire message muse be received by the SMTP server
before any determination of size can be made.

If the client DOES add the SIZE= parameter to the MAIL FROM command
then the SMTP server can reject the MAIL FROM immediately.

Do you have a SMTP proxy or PIX firewall? Does it allow the use of
ESMTP?

>I can see SMTP sessions that goes for hours. Is this the way it works in Non
>Mapi environments?

That depends on whether or not ESMTP can be used, or if it _is_ used,
and if the SIZE= parameter is added to the MAIL FROM command.

>I tried on the server itself configuring OL express and I
>got an NDR straight away.

Then I'd think that the clients that aren't working the same way are
being prevented from using ESMTP. 

-- 
Rich Matheisen
MCSE+I, Exchange MVP
MS Exchange FAQ at http://www.swinc.com/resource/exch_faq.htm

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