You can't fill in any email address as a sender for the mails. Only the senders you add under Settings → Senders can be selected as a sender.
Devisto uses Postmark behind the scenes to deliver all mails. All senders must be verified by Postmark before they can be used as a valid sender.
If you create a new sender and the email address has not been verified before, Postmark will send a verification mail to the address. The link in the mail must be clicked in order to complete the verification process. In Devisto you can check the verification status in the senders table. If you didn't receive the mail, you can always right-click an unverified sender and choose Resend Verification.