SMTP vs. IMAP vs. POP3

Category: Email Hosting · Updated 2026-10-31

SMTP (Simple Mail Transfer Protocol) is used to send and relay email between servers and from a mail client to the server. IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol 3) are the two protocols used to retrieve email that is already waiting on the server; IMAP keeps messages synced on the server across devices, while POP3 typically downloads them to one device and removes them from the server.

What each protocol is for

Protocol	Purpose	Default ports
SMTP	Sending and relaying outgoing mail	25 (server to server), 587 (submission with STARTTLS), 465 (implicit TLS)
IMAP	Reading and syncing mail that stays on the server	143 (plain or STARTTLS), 993 (implicit TLS)
POP3	Downloading mail, usually removing it from the server	110 (plain or STARTTLS), 995 (implicit TLS)

Nearly every mail client setup needs one SMTP connection to send, paired with one IMAP or POP3 connection to receive; the three are complementary, not competing options for the same job.

IMAP vs. POP3: the key difference

With IMAP, messages, folders and read or unread status live on the mail server, and the client simply displays a synced view of them. Check the same inbox from a phone, a webmail browser and a desktop client, and all three stay consistent, including moves, deletes and new folders. With POP3, the client connects, downloads waiting messages to local storage, and by default deletes them from the server afterward, so a second device that connects later will not see those same messages unless the server is set to keep a copy for a limited time.

IMAP is the default choice for anyone who checks mail from more than one device, which today is almost everyone. POP3 still has a narrow use case: archiving mail permanently to one machine while keeping very little stored on the mail server.

Why SMTP needs its own port and authentication

Port 25 is used for server-to-server delivery across the internet and is frequently blocked by residential internet providers and some hosts to cut down on spam sent from infected computers. Mail clients instead submit outgoing mail on port 587 (with the STARTTLS command to upgrade to an encrypted connection) or port 465 (encrypted from the start), always with a username and password. Sending without authentication on these submission ports is rejected by virtually every modern mail server.

Typical mail client settings

Incoming (IMAP):  mail.example.com   port 993   SSL/TLS
Incoming (POP3):  mail.example.com   port 995   SSL/TLS
Outgoing (SMTP):  mail.example.com   port 587   STARTTLS
Username: full email address
Password: mailbox password

Always use the encrypted port and TLS option when one is offered. Plain, unencrypted versions of these protocols still exist for compatibility but send credentials and message content readable on the network.

Troubleshooting common connection problems

Can send but not receive: check the IMAP or POP3 hostname and port, and that the account is not over its storage quota.
Can receive but not send: confirm SMTP authentication is enabled and the correct port (587 or 465) is used, since many networks block port 25 outbound.
Messages missing on a second device: the account is likely set to POP3; switch to IMAP so all devices share the same view.
Certificate warnings: the mail hostname in the client does not match the one on the server's SSL certificate.

Frequently asked questions

What is the difference between SMTP and IMAP?

SMTP is used only to send and relay outgoing email, while IMAP is used only to retrieve and sync email that is already stored on the server. A typical mail account uses both at the same time for different directions of mail flow.

Should I use IMAP or POP3?

IMAP is the better choice for almost everyone today because it keeps mail, folders and read status synced across every device that checks the same account. POP3 only makes sense when you want mail downloaded permanently to a single device and cleared from the server.

What port does SMTP use?

Mail clients normally use port 587 with STARTTLS, or port 465 for implicit TLS, to submit outgoing mail with authentication. Port 25 exists mainly for server-to-server delivery and is often blocked for client connections by internet providers.

Why can I not send email on port 25?

Many home and mobile internet providers, and some hosting networks, block outbound port 25 to reduce spam from compromised devices. Mail clients should be configured to use the submission port 587 (or 465) instead, which requires authentication.

Does IMAP delete email from the server?

No, not by default. IMAP is designed to keep messages on the server and only mirrors what you do, such as reading, moving or deleting a message, across every connected device.

Is it safe to use POP3 or IMAP without SSL/TLS?

No. Unencrypted POP3 or IMAP sends the mailbox password and message contents in plain text over the network. Always choose the SSL/TLS or STARTTLS option, typically on ports 993, 995 or 587, when setting up a mail client.