SMTP sends and relays mail; POP3 and IMAP give clients access to stored mail
Email is not one protocol. SMTP is used to submit and relay messages between mail systems. POP3 and IMAP are mailbox-access protocols used by clients to retrieve or synchronize messages after delivery. A user can therefore send mail successfully while incoming mailbox access fails, or vice versa.
Simple Mail Transfer Protocol moves messages toward destination mail systems
SMTP commonly uses TCP. Port 25 is associated with server-to-server relay, while authenticated client submission commonly uses ports such as 587. TLS can protect SMTP connections depending on configuration. The exact role of a port matters more than memorizing one number for all email behavior.
POP3 is oriented toward downloading messages from a mailbox
POP3 traditionally retrieves messages from a server to a client and can be configured to remove or leave copies on the server. It is simpler than IMAP's server-side synchronization model. Traditional POP3 uses TCP/110; POP3 over implicit TLS commonly uses 995.
IMAP keeps the server mailbox central to synchronization
IMAP is designed for clients to interact with server-stored folders and message state, making it well suited to multiple devices accessing the same mailbox. Traditional IMAP uses TCP/143; IMAP over implicit TLS commonly uses 993.
The conceptual difference is mailbox synchronization, not merely a different port.
What each protocol answers
| Need | Protocol |
|---|---|
| Send a message from a client to mail submission service | SMTP |
| Relay mail between mail servers | SMTP |
| Download/retrieve mailbox messages simply | POP3 |
| Synchronize folders and message state across devices | IMAP |
Swipe horizontally to see the full table.
Email delivery also depends on DNS MX information
Mail servers use DNS MX records to identify mail exchangers for a domain. A healthy SMTP service cannot deliver by domain if the relevant DNS information is missing or wrong. That creates a protocol chain: DNS identifies the mail destination; SMTP carries the mail.
This is a useful example of why protocols should be learned as cooperating services.
Encryption and authentication depend on configuration
Plain legacy ports do not automatically guarantee confidentiality. Modern deployments commonly use TLS and authenticated submission/access. A secure exam answer should distinguish the application protocol's job from the protection applied to its connection.
Use the direction of failure
If users can receive mail through IMAP but cannot send, investigate SMTP submission/authentication/filtering rather than the IMAP server first. If outgoing mail succeeds but the mailbox client cannot synchronize, inspect IMAP/POP access. If mail to one external domain fails while others work, DNS or remote server policy may be involved.
The symptom direction is more informative than the generic statement “email is down.”