use `http.Handler` wrappers to split the sendmail handler into 2
distinct handlers: one for validating the query (currently, checking the allowed
domains) and the other one for the sendmail functionality.
update
use default templates where there are not user-specific settings
improved Dockerfile
renamed healthz endoint
delete deprecated file
fixed healthz endpoint
improved build and deployment
updated docs
Chris Camel
Rodrigue Chakode