Exchange 2007 Hub Transport error “452 4.3.1 Insufficient system resources”

One morning I received a call from the servicedesk  about external mail not arriving in users mailboxes. After sending a test email the reported incident seemed correct. I quickly started an SMTP test from MXToolbox and received the following response from our external receive connector:

EHLO please-read-policy.mxtoolbox.com
250-aaa.bbb.ccc Hello [64.20.227.133]
250-DSN
250-AUTH NTLM
250 8BITMIME [140 ms]
MAIL FROM: <supertool@mxtoolbox.com>
452 4.3.1 Insufficient system resources [5148 ms]
RCPT TO: <test@example.com>
503 Command sequence error – Need MAIL FROM: first. [140 ms]

The Eventviewer didn’t reveal any more information. A Google search showed a possible disk space shortage as cause, but this behaviour was ‘fixed’ in SP1, so probably not. At this time it was pretty clear the Microsoft Exchange Transport service was throwing the error, so why not try the good old restart fix?

To stop waisting your time; A restart of the Microsoft Exchange Transport service fixed the problem.

EHLO please-read-policy.mxtoolbox.com
250-aaa.bbb.ccc Hello [64.20.227.133]
250-DSN
250-AUTH NTLM
250 8BITMIME [140 ms]
MAIL FROM: <supertool@mxtoolbox.com>
250 2.1.0 Sender OK [172 ms]
RCPT TO: <test@example.com>
550 Relaying denied. [156 ms]
QUIT
221 Closing connection. [140 ms]

Have a good one!