If a message is CC'd to you by the sender the reply should go to the sender. If however a message is forwarded to you by one of the original recipients, the sender in that case is that recipient, not the original sender.

It's the email client that sets the reply-to header and it could be that different clients handle it in different ways. Mercury will deliver according to the SMTP envelope provided by the email client.

Hello,

 I have a problem replying to messages that are sent by someone who CCs the message to one of their colleagues.

 When I press reply and send that reply, it is only being sent to the CC and not the sender I'm hoping to reply to.

 How can I fix this

 Thanks for your help!

 (using version 4.63)