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Error:
root@mailgw:~# apt update
Hit:1 http://deb.debian.org/debian buster InRelease
Hit:2 http://security.debian.org/debian-security buster/updates InRelease
Hit:3 http://downloads.metasploit.com/data/releases/metasploit-framework/apt lucid InRelease
Hit:4 https://packages.sury.org/php buster InRelease
Err:4 https://packages.sury.org/php buster InRelease
The following signatures were invalid: EXPKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.sury.org/php buster InRelease: The following signatures were invalid: EXPKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key
W: Failed to fetch https://packages.sury.org/php/dists/buster/InRelease The following signatures were invalid: EXPKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key
W: Some index files failed to download. They have been ignored, or old ones used instead.

Quick fix:
# apt-key del 95BD4743; wget -nv -O - "https://packages.sury.org/php/apt.gpg" | apt-key add -
# apt-get update