[SIZE=2]I would like to generate one ‘Alert’ email for each 850 Envelope received from a specific customer,
containing (for example) the PO# & $Value of each PO, in the body of the email.

Currently each PO in an Envelope received is sent to TN & processed via a processing rule when an X12 4010 850 is received.
I have created an additional rule, executing a new email ‘Alert’ service, when an X12 envelope is received.
This new service calls wm.b2b.edi: envelopeProcess, which returns only the envelope segments.

Is there any way to access the content detail (ie: the actual PO detail)?
(we are running IS: 6.5, TN:6.5 & EDI: 6.5.2)
Any help would be appreciated.
Thank you.
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Yes to retrieve the transactional data (ST level) you need to invoke couple of parsing EDI module services in the following order:

wm.b2b.edi: envelopeProcess (outputs Values document)
wm.b2b.edi.util:getEDIstring (extracts the ST level from ISA/GS map Values outputs EDIstring)
wm.b2b.edi:convertToValues (parse the incoming edidata against EDIFFSchema outputs Document and perform mapping from here)

Please review the EDIModule documentation and service documentation for more info:

HTH,
RMG