Why do messages sent via Gaggle Mail always use an @gaggle.email from address even when I’m using a custom domain?
To ensure successful delivery and that messages don’t get marked as spam we use DKIM and SPF which validate that emails are being sent by who claims to be sending them.
In order to comply with these checks, we have to prove that we are authorized to send from a particular domain. Since we own the @gaggle.email domain we are authorized to send from it and pass the checks, however, if we start sending emails from a @gmail.com or @yahoo.com address then they would not pass the checks and most likely get flagged as spam.
Even though we don’t use other domains as the from address in messages we do use them as the reply-to address. Therefore when someone clicks reply to a message with a custom domain then the reply will go to the custom domain address.
If you host your domain with us your messages will use your custom domain as the from address. This is because we setup DKIM validate for your domain and so we are free to send from it.