Decent article: https://www.fehcom.de/qmail/smtpauth.html

    bash-2.05b$ printf "test" | base64 -e
    dGVzdA==
    bash-2.05b$ printf "testpass" | base64 -e
    dGVzdHBhc3M=
    bash-2.05b$ printf "\0test\0testpass" | base64 -e
    AHRlc3QAdGVzdHBhc3M=