Can be done via IMAP Settings below

 

Settings for IMAP Configuration:

  EMAIL ADDRESS: sharedmailbox@domain.com (shared mailbox)

  IMAP SERVER: outlook.office365.com Port 993 with SSL
  SMTP SERVER: smtp.office365.com Port 587 with TLS

  USERNAME: user@domain.com\sharedmailboxalias  (user\shared mailbox alias)
  PASSWORD: UserPassword (user’s password)

SMTP LOGIN IS DIFFERENT (!)

  USERNAME: user@domain.com (users email)
  PASSWORD: UserPassword (user’s password)

Tags: exchange, IMAP, Mac Mail, Office 365, Secondary, Secondary Mailbox, Thunderbird

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