I sometimes send mails to multiple recipients using the BCC option using various mail client software applications, amongst which, Outlook, 2 emClient installations (A and B for differentiation), multiple EVOLUTION mail clients under Linux, FairEmail for Android - all using the IMAP protocol to attach to the same provider and folders.
Yesterday I checked one of those mails sent from emClient (A) to various BCC addresses using my FairEmail app to double-check who I had sent it to and could not see any of the addresses. Neither could I see the addresses from any other client - including emClient (B)! Calling up the same message using emClient (A) from where I sent the mail originally I can indeed see the BCC addressees. I tried other mails with the same results. Note that BCC mails sent from my other, i.e., non-emClient mail applications show the addresses properly on all clients, including emClient.
I opened the mail header of the messages originated on emClient (A) on all of the other clients. No BCC addresses are being shown except on the machine where the original mail is obviously stored locally.
It seems that emClient stores the message without the BCC address information on the IMAP server (Sent Messages folder) which is at least different from all the other clients I use.