Error 503

Hi All.

My question regards one machine in domain which is having email rejected by 'mail.btconnect.com'.  Setup is via Exchange on SBS2008.

Other users are successfully sending to same external recipients without issue. Problem solely applies to external addresses. All mail is routed through BT.

Please find a brief extract of the error received by the user:

#503 Must enter MAIL command first ##

Thanks all for your help in advance,
SparkyBoy.

LVL 3
SparkyBoyAsked:
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LaserSpotCommented:
It seems like something is corrupting the SMTP conversation between SBS2008 and BT. Does your firewall have any SMTP proxy feature that could be turned off?
Do you have AV software on the server that could be intercepting SMTP?
Is the user able to send e-mail if he uses OWA instead of Outlook?
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SparkyBoyAuthor Commented:
LaserSpot:

These are good points.  Let me check and post a response.

SparkBoy.
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SparkyBoyAuthor Commented:
Having thought that this issue was resolved, it has now returned. In response to comment above, as it now stands, users cannot send via owa. It does work however if i configure the connector to send via dns as opposed to via a smarthost (this is not a preferred option). Firewall has been set to enable smtp.
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SparkyBoyAuthor Commented:
Gave some direction in solving problem.
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