In order to understand how Data Protection displays event participants you must first understand how mail flows are managed.
When an email is created it can comprise of entries for TO: CC: or BCC: TO and CC: entries are visible to the recipient but the BCC are not. In the context of Microsoft Exchange as the mail server host, the mechanics of this process are as follows:
When an Exchange Server encounters an events the MTA (Message Transfer Agent) breaks the message down into two recipient lists. P1 Recipients (The Actual Recipients) and and P2 Recipients (The Display Envelope). The P1 recipient list is used for mail delivery and the P2 recipient list is displayed to the recipients. If the event was captured at the point of processing you could assume that P1 = To+CC+BCC, however mail flow is often much more complex.
You compose an email entering Alice as a To, and Bob as a Bcc.
The P1 recipients at that point were simply Alice and Bob.
When you clicked “Send”, the email goes to ACME’s mail server, which promptly adds your boss as a recipient, and also the journal mailbox (for archiving).
Now your P1 recipients are Alice, Bob, James, and ACMEJournal.
Because Alice is in a different company, at that point the email ‘forks’: a copy of it goes to Unipraxis, while another copy stays at ACME.
The P1 recipients on the copy going to Unipraxis is only Alice while the P1 recipients on the copy staying at ACME are now Bob, James, and ACMEJournal.
When the mail server at Unipraxis receives the email for Alice, it adds UXJournal as a recipient (for archiving). The P1 recipients now on that copy of the email are Alice and UXJournal.
When James gets the email, he will see that you sent an email to Alice. He won’t know about Bob!
When Alice gets the email, she’ll only see herself as a recipient.
When Bob gets the email, he’ll see that it was sent to Alice, but won’t see himself as a recipient. Although he will guess that you sent him a copy as a BCC recipient.
If the email administrators of ACME and Unipraxis have a look in their respective journal, they’ll simple see an email from you addressed to Alice.