I'm using App Engine to send text messages to users (by sending e-mails to their SMS/MMS gateways). The e-mails act essentially as push notifications for iPhone users (since Apple doesn't support Web Push at this time).
The plan is to allow users to also reply to those messages and tweak their subscription (for example, replying with "unsubscribe" would remove their subscription and stop sending them text messages without having to log in to the site and make changes in the "account" screen).
I am sending e-mails using a From: address of notification@XXXX.appspotmail.com, where XXXX is my App Engine project ID.
If I send an e-mail to an AT&T subscriber at NNNNNNNNNN@mms.att.net (or my personal e-mail addresss ZZZZZZ@gmail.com), the sender appears exactly as I originally sent it, and those users may reply to the message (and it is received by App Engine, since it is of the proper format for App Engine to proces it).
However, when App Engine sends an identical e-mail to NNNNNNNNNN@msg.fi.google.com, the sender (the address in the From: line) is always rewritten to be XXXX@appid.googleusercontent.com (where "XXXX" is the App Engine project ID). I don't have any access to the mail for that e-mail address, and it is not posted to App Engine.
When sending mail from App Engine, it adds the following MIME headers:
X-Google-Appengine-App-Id: s~XXXXX-Google-Appengine-App-Id-Alias: XXXX
My guess is that there's something on the Project Fi MMS gateway that's doing some translation that it shouldn't be doing, using those headers.
I couldn't find any useful references to appid.googleusercontent.com online, so I thought that I'd post here.
I checked in with Project Fi's support, and this was not in their playbooks or documentation at all.
So, two questions:
- Why are e-mails sent from App Engine to Project Fi having their
From:address rewritten? - Is there anything that can be done to fix it?
