Buffer Functions (Polling)
Positive Intermediate Reply
331 User name okay, need password.
Transient Negative Completion Reply
421 Too many attempts, closing control connection.
425 Can't open data connection.
426 Can't build data connection: Connection refused.
426 Transfer aborted. Data connection closed.
Permanent Negative Completion Reply
500 Invalid userid/password.
500 Syntax error (command unrecognized.)
501 Syntax error (invalid argument.)
502 Unknown help command.
530 Please login with USER and PASS.
530 Already logged in.
530 TYPE is not supported.
550 File can't be retrieved, partition set by
550 File can't be deleted, partition is not set.
553 File doesn't exist.
553 File is empty.