Have set up alias identity in Thunderbird, but when recipient of email presses 'Reply All' it shows my other identity email

Hi I have set up a default alias identity (call this E2) in my Thunderbird email account. But when I send a message using E2 in the 'from' box, when the recipient comes to reply and selects 'Reply All', my other identity (the actual main name of my mailbox, call it E1) is shown in the CC/BC box. I do not want my other main identity (E1) email showing at all when they reply, just E2. Can anyone advise? Thank you very much.