If I receive an email at an alias address included on the BCC, and I reply to that message, the compose window defaults the From: address to my top-level / account username address.
This behavior is different when replying to a message on which the alias was included in the To: line on the message being replied to. In that case the compose window defaults the From address to the alias that received the message being replied to. This is preferable and I would love to see my replies to BCC messages behave this way too.
I’m really sorry that nobody has commented on this yet.
I believe the problem here is that when replying to a message where the alias is in the To: line the interface can look at the To: header address and then choose the correct alias. However, with BCC: there is no header for that and the alias does not appear in the To: line. Therefore this information is not stored in a way that the interface can select the correct reply from address.
It’s something worth looking at though so I’ll pass this on to our development team.