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How to find the USPS's "Postal Service Mail.dat Technical Specifications" with Mail.dat file/field information and PostalOne! 4-byte error codes in MailStream Plus

To find the USPS Mail.dat Technical Specs  (These instructions are current as-of August 2015)-
Go to ribbs.usps.gov

On the left, open the 'Major/Minor Release Schedule' tab.

Click the year you are interested in, such as '2015 Releases'.

On the next screen, on the left, locate the month's PostalOne! version you are interested in (commonly this will be the latest one.)  In that section click the 'Technical Specifications and Guides' link.

Near the top, there will be a section called "Postal Service Mail.dat Technical Specifications:"  Click the PDF link for that document.

The document contains the USPS regulations for the Mail.dat files that are uploaded to PostalOne!.  For example, you can go through the requirements for each Mail.dat file/field, or if you get a 4-byte error number from PostalOne!, you can do a search for the number and find the recommended action to take.
UPDATED:  August 11, 2017