Google App Engine currently limits you to 2,000 emails per day (for free) via their API.
I am trying to find a definitive answer if it is possible to use a third-party system if you need to send more. I know that they disallow raw sockets, so I would assume that there might be trouble with this approach... but surely I'm not the first to see it.
Worst case, I can build a simple offsite web service that my GAE can call... but I'd much rather just be able to send directly through an SMTP server.
Thanks!
Nope.
You're correct: you cannot make raw socket requests, nor any other direct outbound requests except through the urlfetch API. To talk to an external SMTP server, you would need to use a webservice as a proxy.
We use the Postmark mail outsourcing service via the hutools.postmark API. Since the communication is HTTP based, it works like a charm on Google AppEngine. This might be an option for you, although it is also a for-pay service. We use it to get arround GAEs sender restrictions.
I've successfully used third party providers for email services with Google App Engine. I've used both SendGrid and MailGun using their HTTP-API.