On February 15, 2019, the Postal Service issued a reminder that the upcoming PostalOne! System Release will result in a new required Mail.dat client version:
On March 17, 2019, PostalOne! System Release 188.8.131.52 will result in a new required Mail.dat client version. Users should download Mail.dat client version 184.108.40.206_PROD from the Business Customer Gateway (BCG) using the following path: Mailing Services è Electronic Data Exchange [Go to Service] è Mail.dat download (Windows 32-bit, Windows 64-bit or Solaris).
The updated DRAFT Pre-Release Notes document (Change 3.0) with details on the March 17, 2019 release is posted to PostalPro: https://postalpro.usps.com/march17releasenotes
Please direct any inquiries or concerns to the PostalOne! Help Desk via eMail (firstname.lastname@example.org) or telephone (1-800-522-9085).