Email sending isn't working on Google App Engine though no errors in logs

Question

[I have looked at all the questions related to this topic on SO but neither of them seems to explain the behaviour I am seeing].

I have two projects A (3-4 months old) and B (0.5 month old) and both use a common class to send email on Google App Engine. Both projects are currently running in the free quota.

When I invoke the common Emailer class from Project A, the email gets sent; invoking from B does not produce any errors in the log file (the logs show correct information in email fields), but the email never arrives at the recipient address. I have checked Spam folder/label and even searched entire mailbox for mail from:appspotmail.com (see below) and nothing.

I have verified that for each email sent, the email quota on Developer Console (at console.cloud.google.com/appengine/quotadetails?project=project-b&moduleId=default) registers another recipient, so obviously Google code is being invoked correctly (the email in question is for the reset password, so has only recipient).

I tried sending as admin@mydomain.com (admin@mydomain.com has been added to list of Email API authorized senders on the settings page of app B) and also as admin@project-b.appspotmail.com and neither one works.

On project A so far I have always sent email as admin@project-a.appspotmail.com (where project-a is the id of my GAE app) and have never faced any issue of this sort.

What is going on here? Is there some additional setting I am missing out on?

NB: This probably doesn't have any bearing, but mentioning in the interests of completeness: the recipient email address is of the form myaccount+xyz@gmail.com instead of the more common myaccount@gmail.com. However, manually sending email to this address from within Gmail works, so that shouldn't be an issue.

Update 1

Based on suggestions here and things that I've tried since posting this question, here are some updates:

• I set up an email bounce handler as suggested but it hasn't been invoked yet (as per the logs).

  Maybe it'll be invoked after waiting 24-48 hours. Let's see.

• Set up account with email simplename@gmail.com and exactly the same thing (i.e., nothing) happened when I invoked Forgot password functionality.

• Changed sender email to admin@project-name.appspotmail.com (instead of project-id) and got an Unauthorized sender message in an exception.

  This means that the other addresses I have tried have been authorized senders as I never saw this exception with them.

Answer 1

The best way to troubleshoot App Engine Email delivery problems is to Configure Email Bounce Notification. As Emails are delivered asynchronously, any delivery problems will result in a bounce to the specified sender.