I sent in a support request and received this message:
In order to receive the 100 emails per day, you'll need to choose the
Email API trial or the Marketing Campaigns trial. You can do so by
navigating to Settings > Account Details > Your Products > Start Trial to Send More.
If you start the trial you will receive 40,000 emails a month, but
I have Gandi.net host a personal domain for inbound email, and use my personal domain address as the Reply-To address. However, gMail would sometimes bounce mail as you repost.
I solved it by lowering security in Gmail, following instructions (IIRC) I found at Google. which I am having trouble finding now. There is an active case https://support.google.com/...