Authentication and Deliverability
SPF, DKIM, and DMARC, sending limits, and bounce troubleshooting.
Authentication error: Plaintext authentication disallowed
Email bounce: Domain has exceeded the maximum defers
SPF, DKIM, and DMARC configuration to validate your emails
Email Sending Limit
Mail Protocols – POP3, SMTP, and IMAP
TLS 1.2 on Windows 7, 8, and 8.1
Email bounce: Message blocked
Review incoming and outgoing email logs