问题
I am working on a email client written in C# and as part of this I wanted to be able to have a folder system for storing emails. Does the POP3 protocol support a folder system?
If it does, which command is it?
回答1:
No. A short look at the specifications would have shown you. SMTP does not deal with folders, and POP3 is just a protocol to get an SMTP buffer from a server. Folder management came way later and uses other protocols.
回答2:
POP3 itself does not expose a folder system, but it was invented at a time when a folder was considered a mailbox. Therefore, some systems support a <username>#<mailbox> login. Afterlogic actually has a good explanation here:
http://www.afterlogic.com/support/tutorials-mailbee-net/18-pop3-folders.asp
Basically, on the few systems that support it, to get messages for another "folder" you relogin with a different folder name.
回答3:
No, POP3 does not support a folder structure outside of Inbox, Sent, etc.
You can reference RFC 1939 for POP3.
If you need a folder structure and sycing you should use IMAP4 (RFC 3501).
来源：https://stackoverflow.com/questions/6067618/does-the-pop3-protocol-support-a-folder-system