When emailing your contacts from outside of Upsales there is a way to make these emails show up in Upsales.
This is done by adding email@example.com in the CC field of your email. This way the contact you are emailing won't see the extra recipient.
For emails sent to firstname.lastname@example.org to be handled correctly, they need to be sent from a Upsales user with that email address set in the profile. Your recipient won't know about this even if your sending email is not a correct one.