Currently in the FAQ section of sogo.nu website only installation instructions up to Debian 10 Buster are included.
As Debian 11 Bullseye is stable since a year now, and configuration has slightly changed, it should be added to
https://www.sogo.nu/support/faq/how-to-install-sogo-on-debian.html

When installing repositories with authentication in Debian 11 Bullseye you have to edit 2 files.

1) enhance /etc/apt/sources.list or add new /etc/apt/sources.list.d/sogo.list with

deb https://packages.sogo.nu/release/5/debian/ bullseye bullseye

2) add new /etc/apt/auth.conf.d/sogo.conf with

machine packages.sogo.nu/release/
login   <username>
password        <password>