"send as" function

Currently, mailbird can send emails only with existing accounts in the "from" field. There should be an option to manually enter the "from" address due to using aliases for instance.
Let say I have account jonh@mailserver.com and I have office@mailserver.com alias which has a few employees accounts linked. I would like to send the message to my customer, but I want to use an alias in the "from" field so that when he will use the "reply" button the answer will go to the alias instead of the account which I have used to send him the e-mail.