Answer by Giam for Edit user's email field in Samba domain

Here is something that would work if someone is looking for the solution

cat <<EOF | sudo ldbmodify -H /var/lib/samba/private/sam.ldbdn: CN=Administrator,CN=Users,DC=sub,DC=example,DC=comchangetype: modifyreplace: mailmail: admin@example.comEOF

https://wiki.samba.org/index.php/Administer_Unix_Attributes_in_AD_using_samba-tool_and_ldb-tools