In fact, if you use the From field to specify one of your contacts or you
enter your own value here, the result is "xxx sent on behalf of yyy". The
From header will have this value but the Sender header will show the e-mail
address that you specified in the account that you defined in Outlook.
Account's e-mail address: you@domainA.com
From field's value: them@domainB.com
Recipient might see: them@domainB.com sent on behalf of you@domainA.com
Even if their e-mail client does not reveal the inconsistency, the headers in
your e-mail will have:
So you will not be lying to the recipient as to who really sent that e-mail.
If you want to lie, you will have to edit the E-mail field in the e-mail
account that was defined in Outlook (but recipients can still see your
sending mail server to know the originating domain, and some use SPF which
can be used to track through the logs to see just who was the original
account used to send the e-mail assuming the sending mail server does not add
that info itself in another header).