I was trying to get a send port working in a BizTalk app the other day, but it kept failing with the following message:

Event Type: Error
Event Source: BizTalk Server 2006
Event Category: BizTalk Server 2006
Event ID: 5754
Date: 2/5/2008
Time: 6:06:56 PM
User: N/A
Computer: ACME31
Description:
A message sent to adapter "FILE" on send port "ACME.B2B.ResaleReport.Port.Snd.Http.OneWay.Test.eBI.Steve" with URI "C:\Dev\ACME.B2B.ResaleReport\OneWayPorts\DB2\%MessageID%.xml" is suspended.
Error details: The system cannot find the file specified. (Exception from HRESULT: 0x80070002)
MessageId: {4E99E4FF-5E09-441E-8AB2-2A83FB49B3A6}
InstanceID: {0BFBB93E-3352-4354-A1C7-A153FE90A42C}

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When I removed the map from the send port, I was able to get a message out with no errors.

I was stumped until I tried to run the map inside of Visual Studio, and then I saw this message:
Invoking component...
C:\Dev\ACME.B2B.ResaleReport\ACME.B2B.ResaleReport.Transforms.Premier\ResaleReport_To_EDI867.btm: error btm1067: The external assembly with the fully qualified name "ACME.B2B.ResaleReport.Utilities, Version=1.1.0.0, Culture=neutral, PublicKeyToken=ffee3e80c6a877dd" cannot be invoked. Make sure this assembly is in GAC.
Test Map used the following file: as input to the map.
Test Map success for map file C:\Dev\ACME.B2B.ResaleReport\ACME.B2B.ResaleReport.Transforms.Premier\ResaleReport_To_EDI867.btm. The output is stored in the following file:
Component invocation succeeded.

From this, it was easy to see that there were some custom functoids in the map that were not deployed to the GAC. I deployed the DLL containing the functoids, and the error was fixed.