Determine postbox of incoming eMail

I have a problem that I just can't get a handle on.
I use MS-Outlook to process eMails and check various things via NewMail and NewMailEx in the module ThisOutlookApplication.
My Outlook contains a total of 5 mailboxes. One is an Exchange account and the other 4 are IMAP accounts.
But what I don't succeed with is to determine the mailbox to which the incoming eMail was sent.
For example, when an email arrives, then I would like to know to which postbox it has been sent. To the Exchange mailbox or to one of the IMAP mailboxes ...
I would be very grateful for support and help!
All constructive comments are welcome ...