Email

When you hit Send, your email client (MUA) hands the message to your SMTP server (Simple Mail Transfer Protocol), which verifies the sender and queues the message.

The SMTP server then performs a DNS MX record lookup to find the recipient's mail server, connects to it over the network, and transfers the message.

The recipient's server runs spam/virus checks and authentication (SPF, DKIM, DMARC) before delivering the email to the recipient's mailbox.

Finally, the recipient's client retrieves the message using IMAP (keeps a copy on the server, syncs across devices) or POP3 (downloads to one device and typically deletes from the server).

Protocols by Stage

• SMTP - Sends/transfers mail between servers

• IMAP - Retrieves mail; keeps it on the server

• POP3 - Retrieves mail; downloads and removes from server

Email travels as TCP/IP packets across the internet, and each message is structured with a header (From, To, Subject, Date, routing info) and a body (the actual content, in MIME format for attachments).

Microsoft Exchange Server

Is Microsoft's enterprise-grade mail and calendaring server system that many organizations use instead of generic SMTP/IMAP setup. It runs exclusively on Windows Server and has been around since Exchange 4.0 in April 1996.

• Shared calendars, contacts, and tasks synced across all of a user's devices

• Unified mailbox one central store that all clients (Outlook, mobile, web) read from via MAPI/HTTP or Exchange ActiveSync, rather than downloading copies

• Distribution lists, resource scheduling, public folders, archiving, and compliance (retention policies, eDiscovery, journaling)

• Tight integration with the rest of Microsoft 365 / Office

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