Email concepts
This section discusses various concepts that evolved in the history of email, and which relate to structured email.
Besides topics listed in the following, there are several RFCs which define additional concepts for more specific use cases.
Email filter rules
Email templates
Various email clients allow to create “email templates” or “canned replies”, which provide certain (mostly textual) structure when composing a new email.
HTML email
HTML email provides a more “rich” user experience when reading email messages. Typically embedded in a multipart/alternative body part (see also MIME, email clients can gracefully fall back to a text/plain represention, in case they cannot render HTML email.
From a content perspective, HTML emails often leverage HTML elements such as tables, which offer an easy way to structure email content sent to a user.
Mail-in APIs
See Mail-in APIs
MIME
See MIME
Remote images
HTML email (see above) can include references to external webservers, which the email client loads dynamically once the user opens an email for reading. These so called “remote images” can be used to emulate some dynamic content behaviour (as they can be updated after intial delivery), but are also used to track users.
Many email clients will ask users for special permission before loading remote images (see also following section).
Trusted senders
Trusted senders (may also be called allowlists or whitelists) may exist on client-side to let users define which email senders they trust.
Roundcube Webmail for example, maintains a dedicated address book “trusted senders” which is the basis for deciding the display of remote images.
Terms describe here might be used differently across systems and settings and might need further refinement.