My Inbox had been awfully quiet for a few days. Time to investigate:

Putty with manual SMTP session:

220 <servername> Microsoft ESMTP MAIL Service ready at Wed, 2 Dec 2009 12:53:01 +0100
ehlo server1.nowhere.com
500 5.3.3 Unrecognized command
ehlo server1.nowhere.com
250-<servername> Hello [<client IP>]
250-SIZE
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-STARTTLS
250-X-ANONYMOUSTLS
250-AUTH NTLM
250-X-EXPS GSSAPI NTLM
250-8BITMIME
250-BINARYMIME
250-CHUNKING
250-XEXCH50
250-XRDST
250 XSHADOW
mail from:test@nowhere.com
452 4.3.1 Insufficient system resources
rcpt to:morgan@simonsen.bz

That’s not good. What does the event log say:

Event Log:

Index              : 5950
EntryType          : Error
InstanceId         : 3221502622
Message            : Microsoft Exchange Transport is rejecting message submissions because the available disk space has
dropped below the configured threshold.

                     The following resources are under pressure:
Queue database logging disk space (“C:Program FilesMicrosoftExchange ServerV14TransportRoles
dataQueue”) = 88% [Medium] [Normal=86% Medium=88% High=90%]
Physical memory load = 95% [limit is 94% to start dehydrating messages.]

                     The following components are disabled due to back pressure:
Inbound mail submission from the Internet
Mail submission from Pickup directory
Mail submission from Replay directory
Content aggregation

                     The following resources are in normal state:
Queue database and disk space (“C:Program FilesMicrosoftExchange ServerV14TransportRolesdata
Queuemail.que”) = 88% [Normal] [Normal=92% Medium=94% High=96%]
Version buckets = 0 [Normal] [Normal=80 Medium=120 High=200]
Private bytes = 8% [Normal] [Normal=71% Medium=73% High=75%]
Batch Point = 0 [Normal] [Normal=1000 Medium=2000 High=4000]

Category           : ResourceManager
CategoryNumber     : 15

Source             : MSExchangeTransport
TimeGenerated      : 29.11.2009 21:45:58
TimeWritten        : 29.11.2009 21:45:58
UserName           :

That’s worse. The server had about 1.6 GB free disk space, and had hit the threshold set for the Queue database. I extended the disk to 32 GB and all was well. Hopefully not too many messages have been lost. SMTP has a default retry period of 48 hrs so I should have a lot of new messages as the various sending servers retry to deliver their messages.