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This link:
https://www.xfinity.com/support/articles/email-port-25-no-longer-supported

Explains why Xfinity, Apple iCloud, and other email services NO LONGER SUPPORT SMTP email ports 25 and 110, for important security reasons.

Instead, for years now, they have required using port 587 for SMTP.

But the Arris SBG6900-AC gateway router firewall’s Medium and High security settings exclude port 587, forcing users to keep their firewalls always set at the LOWEST security level.

This is unacceptable, and I’m surprised Arris hasn’t fixed this deficiency.

Arris SBG6900-AC users deserve a control program update and/or firmware update, to enable port 587 in the Medium and High firewall security tables.

 

Thank you for the information about the Firewall settings.

Whether the firewall is set to the lowest or the highest setting, the firewall provides the same level of stateful packet inspection (SPI). The medium and high settings also use port blocking that can block a lot of commonly used ports. Most of the time using medium or high isn’t needed, as SPI and other features such as NAT will offer the necessary protection. If you are in a very sensitive environment you may want to use medium or high to block other ports. 

We will look into the updated SMTP port list. 

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