When building a new application we found the sample templates from postmark published by Postmark provide a good start point. For each template a clear purpose is addresses and there is a good balance of information to include in each message.
caniemail.com for checking support of HTML features.
Get an aggregated report of your delivery. Later setup up denial of the emails.
Report Analyzer by MX Toolbox.
SPF | Sender Policy Framework
Document the server whom are allowed to send emails on the behalf of your domain. A TXT record is placed on the DNS of the domain following a syntax