With the help of Outlook username and password, log in to your account.
Enter the options of "settings".
With the help of drop down menu options, choose the connected accounts.
Click the options of other email account under the options of related account.
Enter the email address and password of your Rackspace email account.
Enter the button of OK.
Choose the options of IMAP and link now.
Enter the button of finish.
Read more here:- https://www.datarecovo.com/setup-and-sync-rackspace-email-with-outlook/
How to Configure My Email Client with Rackspace Mailbox
Your Name: Your Name or Business Name.
Login Name: Your Mailbox Email Address.
Password: Your Mailbox Password.
Account Type: IMAP.
Incoming Mail Server: secure.emailsrvr.com.
Outgoing Mail Server: secure.emailsrvr.com.
Incoming Mail Server (IMAP) TCP Port: 993 (SSL)
Outgoing Mail Server (SMTP) TCP Port: 465 (SSL)
Regards,
Caleb