Is it possible to send notifications to local mailboxes of the users (i.e., /var/spool/mail/$USER) via Linux mail utility in GitLab?
Version of the running instance is GitLab CE version 16.3.5.
I noticed that I can do it by adding a new e-mail address to my local GitLab account. Format of the e-mail address should be username@localhost. Then you can use an e-mail client to read the e-mails sent by the local GitLab instance as explained in https://askubuntu.com/a/22715.
Not sure if it is required, but you may also want to refer to https://docs.gitlab.com/omnibus/settings/smtp/ to manage the GitLab configuration for sending e-mails.