All input from network sockets is checked for ASCII NUL ('\0') characters and the connection is immediately and silently dropped if a NUL character is found. This event is logged as message 0x0035 ('null_byte_in_input'). A NUL byte can never be part of a valid POP3 dialogue.
pserv and pcheckd check the mailbox names coming from pproxy. Only the following characters are allowed in mailbox names: a-Z, A-Z, 0-9, '.' (dot), '_' (underscore), '-' (hyphen), '+' (plus), '/' (forward slash), '=' (equal sign), and '%' (percent). Two adjacent dots ("..") are not allowed.