I just ran into this problem today, with a Plone 5.0 site. I configured it in the Mail settings panel to send email using a Gmail account. The account does not have 2 factor authentication enabled, and I know I entered the correct password. The error displayed when I used the "Save and send test e-email" button was:
Unable to send test e-mail (534, '5.7.14 https://accounts.google.com/ContinueSignIn?somehorrendouslylongURL Please log in via your web browser and\n5.7.14 then try again.\n5.7.14 Learn more at\n5.7.14 https://support.google.com/mail/answer/78754 133sm7809684qhu.9 - gsmtp').
That page at https://support.google.com/mail/answer/78754 mentions "Your app might not support the latest security standards".
The page at Less secure apps & your Google Account - Google Account Help describes Gmail's tightening of security. I've enabled the "Allow less secure apps" setting but am still not able to send out email from the site.
Any thoughts on how we can make it less irritating to use Gmail for outbound emails from Plone?
UPDATE:
The first time I enabled "Allow less secure apps" it did not stick. I reloaded the page and tried again and it worked, and I was able to send out email from the Plone site.