I created an Outlook Add-In that reads the Recipients on Item Send and
take actions based on the Recipient's MailBox ID. However, we are
getting inconsistent Mailbox IDs for some users.

I created some VBA code to test this. Here's what's happening:

On Item Send, Add-In reads RecipientA and his Mailbox ID is 101.

With RecipientA logged into Outlook, Add-In reads his MailBox ID =
102.

This doesn't happen on all PCs. Only on some.

Anybody has an idea why this is happening?