Basic Email Components (To, From, Subject, Body)

What does the "To" field mean? What does the "From" field mean? Why is the subject line important? What is the email body? What is CC (Carbon Copy) in email? What is BCC (Blind Carbon Copy) in email? When should I use CC vs. BCC? What is an email signature? What is the “From” header and what does it mean? What is the “Reply-To” header? What is the “Return-Path” header? What is the “To” field? What is the “CC” field? What is the “BCC” field? What is the “Subject” line? What is the “Date” header? What is the “Message-ID”? What is the “In-Reply-To” header? What is the “References” header? What is the “MIME-Version” header? What is “Content-Type”? What is “Content-Transfer-Encoding”? What is the difference between plain text and HTML body parts? What is the “boundary” parameter in MIME? What is multipart/alternative vs multipart/mixed? What are inline attachments? What is an attachment disposition? What is “Content-ID” and how does it reference inline images? What is a header vs. envelope header? What is header folding? What is a header field limit? What is an invalid header? What is header injection? What is subject encoding (=?UTF-8?...=)? How do emojis appear in subject lines? What are common formatting pitfalls in email headers? How does case sensitivity work in email headers? What happens if multiple “From” headers exist? What is a malformed message? What is the difference between HTML body and text body? What is a base64-encoded body? What is line length limit in an email body? What is quoted-printable encoding? What is charset UTF-8 vs ISO-8859-1? What is HTML sanitization in email clients? What is remote content blocking? What is an image proxy? How do email clients render external images? What is a tracking pixel? What are CID images? What is inline CSS vs embedded style block? Why do some clients strip CSS? What happens if an email has no body? What happens if an email has no subject? What is the difference between header fields and metadata shown to users? Why do some emails show a different “display name” than the actual address? What is a display name spoof? What is “friendly-from”? What is address internationalization (EAI)? What is an email alias format (plus-addressing)? What is “reply-to-all” behavior? What happens when you forward an email? What changes when you reply? What headers are preserved when replying? What happens when you resend an email? What is the difference between “resend” and “forward”? What metadata gets stripped by mail clients?