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Last tested: Nov 30, 2020
When we see this error message on SMTP setting page, more likely to be due to incorrectly defined the "from" setting. User should insert an email address for "from" setting.
ref:
- Email With Rails 3 and Heroku Net::SMTPSyntaxError: 501 Syntax error
- "Sender syntax error" in Chilkat4Xojo
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