In the Gmail settings, under "Accounts and Import", I added a new email address under the "Send mail as" setting. This email address using my own domain, firstname.lastname@example.org, lets say. I've set up Gmail for sending via SMTP servers from Fastmail.
This domain has been configured on Fastmail. The MX records were defined and I added the alias, asking Fastmail to transfer all emails to my @ gmail.com address.
Receiving mail at email@example.com works as expected, the mail appears in my Gmail inbox. Sending from the Gmail web interface as firstname.lastname@example.org also works. However, the recipient receives the email as it comes from my email address @ fastmail.com. How can I solve this problem?