Using SMTP via CURL functions, you can send emails. Check our SendMail functions to build email with/without HTML, multiple attachments and properly encoded names & texts. See this tutorial: Sending emails in FileMaker with MBS Plugin
We can even configure it for Outlook 365, Google mail and others. No problem there.
You an also check the IMAP examples coming with the plugin. See a video here: IMAP Email Example
The examples are included with MBS Plugin.
Direct Mail or Bulk Mail sorting pertains to how the mail pieces are bundled for the post office. Depending on the size and weight of the pieces, there are constraints on how many fit in a tray or flat and those must be bundled in minimum groupings based on zipcode or desination etc. I have done it before many moons ago, but thought someone here has more recent experience.
The mail.dat specification is a set of related fixed-length database text files, of up to 21 tables, that are incorporated into what I now understand is a zipped file for submitting to USPS.
While we wait to "roll our own" mailing system, we will continue to use the current dBase-based, .ini-driven, FoxPro-scripted legacy code.