Getting a SocketError while sending emails on heroku, using sendgrid? It may be some gmail env vars.

While working on a project that I just inherited, we enabled sendgrid for our heroku app. Sending an email raised this error:

SocketError (getaddrinfo: Name or service not known)

Google searches indicated ActiveMailer configuration issues, but Heroku was pretty clear:
Rails apps using ActionMailer will just work, no setup is needed after the addon is installed.

While ensuring that RACK_ENV was set correctly (when all else fails), I found these two settings:

Removing them fixed my email. Sendgrid is working fine.

Armed with this evidence, I found "tech_sending_email_with_gmail"
And so... someone had set up the server for gmail, but something about it was failing now.

Now I know. And you do too.

Note: this is just one cause of this error.