SMTP timeout after sending data block… What’s The Issue?

This type of mail server to mail server SMTP failure is usually accompanied by log message like the following:

“SMTP timeout while connected to xx.xx.xx.xx [xx.xx.xx.xx] after sending data block (8189 bytes written): Connection timed out”

This type of failure could arise from any number of network connection issues, firewall packet inspection, a security application, or a virus scanner on the local mail server.  However, in every case we have encountered this it has been due to an issue with security systems on the receiving system, typically on the router itself.  For the most part, this failure only occurs when the recipient’s scanning system is processing emails with attachments (regardless of size) and it reaches a point where it fails/freezes.  After that, the recipient system stops accepting new SMTP data and after a predetermined time, the SMTP connection then times out.

There is nothing that the sending server can change in order to force the recipient’s firewall and/or mail server to accept such messages.  Further, note that while the connection is in a frozen state, this can prevent emails queued behind the failed message to be delayed until the recipient server eventually times out and a new connection can be opened.

It is possible that adding the sending server’s IP address to a ‘trusted host’ list will alleviate the issue, if such a convention exists on the firewall and/or recipient mail server.

Here are some specific products that we have seen exhibit this behavior in the past, and what actions our clients report as having solved the “SMTP timeout after sending data block” issue:

Microsoft Internet Security & Acceleration (ISA) Servers

  • Under configurations-add-ins-applications, turn off the SMTP filter.

Watchguard Firewall

  • Disable SMTP Proxy filtering

Cisco Firewalls

  • Remove ESMTP inspection
  • Disable Intrusion Detection

We will update his list whenever clients report other solutions to this SMTP error.

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