Suspicious sign in prevented (Heroku, Amazon AWS, Gmail SMTP)

Short Answer

1. In a web browser (perhaps Chrome/Incognito), log into your Gmail account (the one via which you're trying to send email from your app).

2. In another browser tab, open https://accounts.google.com/DisplayUnlockCaptcha -- and follow the steps.

Long Answer

I ended up logging into my Gmail account (mentioned as ________@gmail.com in the Stackoverflow question above) and saw the following warning:

[Note: I'm piecing this Answer together after the fact.] Clicking the link Was it you?, I'm pretty sure I was taken to a page that contained the following, though below it there was a message and button about adding another application to the list:

[Again, I'm pretty sure...] Clicking the button on that page brought me to https://accounts.google.com/DisplayUnlockCaptcha -- which looks like this:

Clicking Continue brought me to another page:

At that point I just went to my app, and made it send an email.

Note: for general live-testing of email in my app, I have a view at triggers#index with two buttons that send PUT requests to these actions: triggers#send_email and triggers#raise_exception. This enables an admin to go to /triggers and send an email or raise an exception to test if the production system is working correctly as far as being able to send email is concerned. Raising an exception in any of my apps emails details to me via exception_notification.

I had the same problem when I tried sending emails through Gmail SMTP using PHP. You shoud complete the troubleshooting steps provided at the end of the email.

On your Gmail mail box look at the bottom right corner and you can see a Details button. Click on it and then on the pop up window change the suspicious warning settings.