Exchange 2007 responding with 499 no file descriptor error

We have a problem receiving mail on our Exchange 2007 server.  Inbound mail is being rejected with the error:

2010-04-28 15:06:02 M=<D68E52DBA02FD34DA0CE008BAF05D35905DF37ED24@SVEXCHCLUSTER.m1.local> SZ=786704 C="499 no file descriptor"

When telnetting to the server we're getting messages like this:

Connected to 62.133.12.68.
Escape character is '^]'.
220 *********************************************************************************************

I'm told the string of *'s is exchanges way of saying something is wrong.

Emails get through after a couple of hours.  We changed broadband supplier a couple of weeks ago. Nothing else has changed.  We use Symantec Mail Security for MS Exchange for anti spam and email virus scanning.
Peter_CullAsked:
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AkhaterCommented:
this is the reply of a PIX firewall

go to the PIX and disable SMTP checking it should work

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Peter_CullAuthor Commented:
Great answer.  My ISP enabled SMTP checking without telling me.  They disabled it and now email is arriving fine. Cleared the backlog in minutes.  Many thanks.
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