POPular implements the 'Post Office Protocol - Version 3' as described by RFC 1939. The optional commands TOP and UIDL from RFC 1939 are supported. The APOP command is not supported.
The POPular implementation differs from RFC 1939 in respect to mailbox locking. Please see the chapter on Mailbox locking for details.
POPular is compliant to RFC 2449 ('POP3 Extension Mechanism'). It supports the following capabilities: TOP, UIDL, USER, and PIPELINING. It does not support SASL, RESP-CODES, and LOGIN-DELAY. Please see the chapter on POP3 extensions and capabilities for details.
POPular does not support the LAST command, which was described in older RFCs, but was removed in RFC 1725 in November 1994 and is not even mentioned in the current RFC 1939.
Of the three possible authentication mechanisms (USER/PASS and APOP described in RFC 1939 and the AUTH command described in RFC 1734 and RFC 2195) only USER/PASS is supported.
Strictly speaking a POP3 server is not allowed (RFC 1939) to answer a connection attempt with a negativ response ('-ERR'). POPular can do this anyway, if you want to take down the server for maintainance for instance. Alternatively you can configure POPular not to accept the connection at all or fake an empty mailbox, which would both be conforming to the RFC.
The POP URL Scheme (RFC 2384) is not needed in POPular.
The following RFCs concerning POP are old and don't apply any more: RFC 918, RFC 937, RFC 1081, RFC 1082, RFC 1225, RFC 1460, RFC 1725, RFC 1957, RFC 2095.
The RFC 2595 ('Using TLS with IMAP, POP3 and ACAP') is supported. See the TLS information in the security chapter for details.