OT: Blocking oversized messages during SMTP transaction

[Jim Holland]

Hi all

Sorry to be a little off topic here, but we have a sendmail issue that I
hope someone might be able to assist with.  It is driving us mad!

We have only very limited bandwidth - 64k for 2,500 (yes) e-mail users -
so need to utilise our bandwidth as efficiently as possible.  We have
therefore configured the maximum message size in sendmail 8.13.1 to 1.5 MB.
This normally works fine with intelligent mail systems that understand 
what "250-SIZE 1500000" means.

However some systems such as Yahoo and, surprisingly, GoogleMail, fail to
use the SIZE extension when sending us mail.  The result is that we accept
large messages from them as we don't know what the message size is going
to be in advance, and then when the specified size limit is reached for
the temporary mail file, sendmail still stupidly insists on continuing
with the transaction even though it will no longer write to disk.  That
means that if Yahoo sends us an 8 MB file we accept the first 1,5 MB and
write that to disk but then keep accepting the remaining 6,5 MB bytes only
to chuck them into the bit bucket.  Only when the transaction is complete
does sendmail inform the sender that the message is too large.  Clearly
this is a total waste of time and bandwidth.

This evening some idiot tried to send us 12 separate copies of a 4,5 MB
message!  Killing the connections doesn't work as Yahoo obviously just
keeps resending.  I had to wait until it became apparent who the sender
was, then blacklist them in the access file and only then kill the
connections.

Is there any automated solution to this?  We need to be able to terminate
the session with an appropriate error message the moment it is apparent
that the message is too large.  Can sendmail be made to issue a fatal
error message during the DATA phase of the SMTP transaction?  Or is the 
crunch time when we need to bite the bullet and change to say Exim instead?

With the increase in size of messages allowed by systems such as Yahoo and 
Gmail this is going to be more and more of a problem for us.  If the above 
messages had arrived during the day they would have killed our bandwidth
completely for several hours.

Any help would be most appreciated - apologies for being off-topic, but I 
know the gurus are here on this list!

Regards

Jim Holland
System Administrator
MANGO - Zimbabwe's non-profit e-mail service

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