"My emailer is Pegasus."
Just repeated the experiment to four addresses.
All addresses are in my Demon subdomain. One To: and three bcc:
Instead of receiving four emails - there are only two.
First one raw view only has the To: field
Second one has To: field - followed by the line "BCC:" - containing all three bcc addresses.
Something is wrong with the Office 365 handling of a header's BCC field - or Pegasus is coding the multiple addresses in a way that is not recognised beyond the first? one.
This the anonymised raw header view - the To: keyword is mixed case - the BCC is uppercase.
Date: Tue, 29 May 2018 15:55:26 +0100
Subject: bcc test2