ok... makes a little more sense now.
I'll try to explain this with a lot of detail... so I will be very verbose.
You are not using Webflow hosting.
Your domain is hosted on an external server... not Webflow
Tracing your domain... your domain appears to hosted with KingHost in Brazil
To send messages from firstname.lastname@example.org
--- you must have a 3rd level domain setup on Webflows servers
--- example would be: facilitasbrasil.webflow.com... (or something like that)
--- where "facilitasbrasil" is the 3rd level domain
--- webflow is the 2nd level domain
--- io is the first level domain
--- Is it possible to change the sender email@example.com to firstname.lastname@example.org
Yes. Your domain is setup with KingHost.
--- You should be able to create an email address with KingHost as "email@example.com."
If you are trying to get Webflow's servers to send the message as "firstname.lastname@example.org."
- they cannot... Unless Webflow is hosting your domain "facilitasbrasil.com.br"
Webflow can send out the message as email@example.com
--- because webforms.io is hosted on Webflows servers
You want to change the "from address".... to "firstname.lastname@example.org".
Since KingHost is hosting your domain (facilitasbrasil.com.br)...
--- you should be able to send a message from "email@example.com" from the KingHost servers
--- but you cannot send a message from KingHost as "firstname.lastname@example.org"
--- because the domain "webforms.io" is not hosted on KingHost's servers.
The server that hosts the domain... is the only servers that can send out email with that domain name
--- unless you are spoofing the domain name (which you should not be)
--- or the server is setup as a relay (which you don't want to do).