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https://wjianz.wordpress.com/2014/09/03/howto-installconfigure-msmtp-and-mutt-on-ubuntu/

I got GTS CA 1O1 as the common name instead of Google Internet Authority G2. What is the difference between the two.

So GTS CA 1O1 refers to the one listed here https://pki.goog/?

I see GTS CA 1O1 valid until Dec 15, 2021. So by Dec 15, 2021, I should regenerate the local crt file by openssl x509 -inform DER -in GTS1O1.crt -outform PEM -out gmail-smtp.crt

$ msmtp --serverinfo --tls=on --tls-starttls=off --host=smtp.gmail.com
SMTP server at smtp.gmail.com ([172.217.195.108]), port 465:
    smtp.gmail.com ESMTP a10sm3703146oic.46 - gsmtp
TLS session parameters:
    (TLS1.3)-(ECDHE-X25519)-(RSA-PSS-RSAE-SHA256)-(AES-256-GCM)
TLS certificate information:
    Owner:
        Common Name: smtp.gmail.com
        Organization: Google LLC
        Locality: Mountain View
        State or Province: California
        Country: US
    Issuer:
        Common Name: GTS CA 1O1
        Organization: Google Trust Services
        Country: US
    Validity:
        Activation time: Tue Nov  5 15:45:23 2019
        Expiration time: Tue Jan 28 15:45:23 2020
    Fingerprints:
        SHA256: 50:E7:13:03:7B:A8:D8:28:3C:D2:66:AC:58:E3:76:6D:BB:DB:E2:9D:B6:8F:54:38:10:BC:A5:93:67:25:7D:4D
        SHA1 (deprecated): F4:D9:49:8F:FA:F0:06:D1:B8:D7:AE:A8:56:A3:36:B4:FB:76:3E:32
Capabilities:
    SIZE 35882577:
        Maximum message size is 35882577 bytes = 34.22 MiB
    PIPELINING:
        Support for command grouping for faster transmission
    AUTH:
        Supported authentication methods:
        PLAIN LOGIN OAUTHBEARER 
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I see GTS CA 1O1 valid until Dec 15, 2021. So by Dec 15, 2021, I should regenerate the local crt file by openssl x509 -inform DER -in GTS1O1.crt -outform PEM -out gmail-smtp.crt

If google at any point changes their certificate, and you statically trust only a single certificate, you will have to refresh it, yes.

The proper way to do this would probably be to use your OS' set of trusted certificates. On Debian they're by default located in /etc/ssl/certs, and google uses certificates that's trusted by almost all operating systems. This would avoid any manual work on your part, but reduce security slightly. The guide you follow tells you how to do this:

tls_trust_file /etc/ssl/certs/ca-certificates.crt

This will make it trust all certificates that your distribution (Ubuntu) includes as trusted.

I would suspect that you have no guarantee that google will return identical certificates on all connections, nor that they won't change them well ahead of expiry time.

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