The problem seems to be that I was sending the emails from my yahoo account, to a google account, but the domain name of my website is not "yahoo."
Google has some new DMARC policy. I think this policy prevents emails from being delivered if the domain name in the "from" field is different from the domain name from which the email was sent.
The following showed up in my spam folder:
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
host gmail-smtp-in.l.google.com [188.8.131.52]
SMTP error from remote mail server after end of data:
550-5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain's
550-5.7.1 DMARC policy. Please contact the administrator of yahoo.com domain if
550-5.7.1 this was a legitimate mail. Please visit
550-5.7.1 https://support.google.com/mail/answer/2451690 to learn about the
550 5.7.1 DMARC initiative. x3si5369085itb.172 - gsmtp