What is error 501 at Forte Agent?

The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol) is an application layer Internet protocol that allows a local client to access e-mail on a remote server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501. IMAP4 and POP3 (Post Office Protocol version 3) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both.