technical · sourced answer
What is POP3? Post Office Protocol Version 3 Explained
POP3, or Post Office Protocol Version 3, is a standard application layer protocol used by email clients to retrieve emails from a remote server over a TCP/IP connection. Unlike IMAP, POP3 is designed to download messages to a local device and typically deletes them from the server immediately after retrieval.
Mechanical Operation
POP3 operates in three distinct stages: authorization, transaction, and update. During authorization, the client provides credentials to the server. In the transaction stage, the client retrieves specific messages using commands like RETR. Finally, the update stage marks messages for deletion and closes the session, committing the changes to the server mailbox.
POP3 vs IMAP
The primary difference is synchronization. POP3 downloads the entire mailbox to a single device, making it ideal for offline access but problematic for users with multiple devices. IMAP syncs headers and bodies in real time, keeping the master copy on the server. POP3 is simpler and requires less server storage but lacks the flexibility of modern cloud based mail access.
Sender and Infrastructure Impact
For senders, POP3 is an inbound protocol and does not affect the transmission of mail via SMTP. However, if a recipient uses POP3 and downloads mail to a single machine, the sender will not see status updates across multiple devices. Infrastructure teams must ensure port 110 for cleartext or port 995 for SSL/TLS is open to allow client access.
Operational Considerations
A common mistake is failing to enable the Leave Messages on Server option in the client settings. Without this, emails are permanently removed from the server after the first download, preventing backup or access from other apps. Modern security standards require using POP3S (POP3 over SSL/TLS) to prevent credential theft during the authorization phase.
Implementation Example
A typical POP3 session begins with the client connecting to the server on port 995. The client sends USER and PASS commands to authenticate. Once authenticated, the client uses LIST to see available messages and RETR 1 to download the first message. After the session ends with QUIT, the server removes the downloaded messages from the spool.
Questions teams ask
Which port does POP3 use?
POP3 uses port 110 for unencrypted connections and port 995 for encrypted connections via SSL/TLS.
Does POP3 support folders?
No, POP3 only interacts with the inbox. It cannot see or manage server side folders like Sent or Drafts.
Is POP3 still used today?
Yes, though IMAP and API based access are more common, POP3 is still used for simple setups and local archiving.
Can I use POP3 with SendHQ tools?
While POP3 is for receiving, you can use SendHQ free tools at https://sendhq.cc/tools to verify the DNS records of the domains you are sending to.