How Apple Push Notification Service Affects iMessage Connection

If you are facing losing connection of iMessage on macOS over a proxy network, please try to add a rule to make Apple Push Notification Service (`+.push.apple.com`) bypass proxy. And for Mail failing to be sent, this is possibly because that proxy service provider's blocking as avoiding spam email detection. Try to add a rule of Apple mail service (`+.me.com`) bypassing proxy. The reference information is listed below:

iMessage Security Overview

iMessage makes extensive use of the Apple Push Notification service (APNs).

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When a user turns on iMessage on a device, the device generates encryption and signing pairs of keys for use with the service. For encryption, there is an encryption RSA 1280-bit key as well as an encryption EC 256-bit key on the NIST P-256 curve. For signatures, Elliptic Curve Digital Signature Algorithm (ECDSA) 256-bit signing keys are used. The private keys are saved in the device’s keychain and only available after first unlock. The public keys are sent to Apple Identity Service (IDS), where they are associated with the user’s phone number or email address, along with the device’s APNs address.As users enable additional devices for use with iMessage, their encryption and signing public keys, APNs addresses, and associated phone numbers are added to the directory service.

If Your Apple Devices Aren't Getting Apple Push Notifications

The connection on port 443 uses a proxy as long as the proxy allows the communication to pass through without decrypting.

Use Apple Products on Enterprise Networks

Apple products require access to the internet hosts in this article for a variety of services. Here's how your devices connect to hosts and work with proxies:

- Network connections to the hosts below are initiated by the device, not by hosts operated by Apple.

- Apple services will fail any connection that uses HTTPS Interception (SSL Inspection). If the HTTPS traffic traverses a web proxy, disable HTTPS Interception for the hosts listed in this article.

Making Apple Push Notification Service Available On Your Network

Question about Working with APNS: Proxies and Ports