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Which SMTP Port to Use for Email: 25, 587, 465, or 2525?
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Networking 101: Transport Layer Security (TLS) - High Performance Browser Networking (O'Reilly)
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Networking 101: Transport Layer Security (TLS) - High Performance Browser Networking (O'Reilly)
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SSL vs. TLS vs. STARTTLS Encryption | Mimecast
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Enabling TLS/SSL Encryption on the SMTP Service | Runbox Help
KB484645: Email delivery fails with error, "STARTTLS is required but host does not support STARTTLS"
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![Guide] Office 365 email notification for DataCenter (using TLS and non-TLS methods) – NovaBACKUP Support](https://support.novabackup.com/hc/article_attachments/360005885218/Example_DC_Email_Encryption_STARTTLS.PNG "Guide] Office 365 email notification for DataCenter (using TLS and non-TLS methods) – NovaBACKUP Support")