Symantec — Email Fundamentals¶
Before configuring any email security product, it helps to know what an email message actually is under the hood — how it's formatted, what protocols move it around, and what the different "body" formats mean for filtering and security.
Types of email body¶
| Type | Description | Use Case |
|---|---|---|
| Plain Text | No formatting | Security alerts, system logs, command-line tools |
| HTML Email | Styled emails with colors, fonts, links, images | Business mail, marketing newsletters |
| Multipart Email | Both a Plain Text and an HTML version included | Best practice for compatibility |
Email structure (RFC 5322 standard)¶
RFC 5322 defines the standard structure of an internet email message: a set of header fields followed by the message body.
Header Section
--------------
From:
To:
Subject:
Date:
Message-ID:
MIME-Version:
Content-Type:
Body Section
--------------
Message content (text / HTML / files)
MIME structure example¶
MIME (Multipurpose Internet Mail Extensions) is what allows a single email to carry multiple content types — for example, a plain-text and an HTML version of the same message, or file attachments — inside one message, separated by a boundary marker.
Content-Type: multipart/alternative; boundary=xyz
--xyz
Content-Type: text/plain
Hello, this is plain text.
--xyz
Content-Type: text/html
<b>Hello, this is HTML version!</b>
How email works (flow overview)¶
Mail Client (Outlook / Gmail / iPhone)
↓ SMTP
Mail Server (MTA: Postfix, Exim, Sendmail)
↓ DNS Lookup (MX record)
Receiving Mail Server
↓ IMAP / POP3
User Inbox
Main email protocols¶
| Protocol | Purpose |
|---|---|
| SMTP (Port 25 / 465 / 587) | Send email |
| IMAP (Port 143 / 993) | Read email (server-based sync) |
| POP3 (Port 110 / 995) | Download email to client |