CiviMail Alternative Outbound Mailbox/From Address
is it not possible to use the mailbox associated with the FROM address when sending mail using CiviMail?


It is possible to send email using different from email address. The expected behaviour is that the email address you choose when composing a CiviMail is the email address that appears to the recipient in their email client.

It sounds to me like your SMTP service is rewriting the from address before sending the mail. That might be because the SMTP credentials you have supplied are only to be used to send email from that specific email account (in which case, it is not particularly well suited for use as a CiviMail mailer).

You might want to check with the email provider to see if that is the case. Or run a test using a different mailer in 'Settings - Outbound Mail'  (e.g. 'Mail', or 'Redirect to database') and see what the From addresses look like when sent using these methods.