Access Exchange shared mailbox in thunderbird

Exchange Configuration

USER EMAIL: user@maindomain.com   
PASSWORD: password
SHARED MAILBOX: shared@anotherdomain.com      (could also be @domain.com, of course)   
SHARED MAILBOX ALIAS: shared-mailbox

Note: Please make sure to create the UNIQUE alias always with the mailbox, because it creates the user [alias]@maindomain.com. In this case shared-mailbox@maindomain.com

Settings for IMAP Configuration

EMAIL ADDRESS: shared@anotherdomain.com (shared mailbox)
IMAP SERVER: outlook.office365.com   
SMTP SERVER: smtp.office365.com
USERNAME: user@maindomain.com\shared-mailbox  (user\shared mailbox alias)   
PASSWORD: password (user's password)

SMTP Login Is Different

USERNAME: user@maindomain.com  (users email)   
PASSWORD: password (user's password)