From RFC2060:

5.1.3.  Mailbox International Naming Convention

  By convention, international mailbox names are specified using a
  modified version of the UTF-7 encoding described in [UTF-7].  The
  purpose of these modifications is to correct the following problems
  with UTF-7:

     1) UTF-7 uses the "+" character for shifting; this conflicts with
        the common use of "+" in mailbox names, in particular USENET
        newsgroup names.

     2) UTF-7's encoding is BASE64 which uses the "/" character; this
        conflicts with the use of "/" as a popular hierarchy delimiter.

     3) UTF-7 prohibits the unencoded usage of "\"; this conflicts with
        the use of "\" as a popular hierarchy delimiter.

     4) UTF-7 prohibits the unencoded usage of "~"; this conflicts with
        the use of "~" in some servers as a home directory indicator.

     5) UTF-7 permits multiple alternate forms to represent the same
        string; in particular, printable US-ASCII chararacters can be
        represented in encoded form.

  In modified UTF-7, printable US-ASCII characters except for "&"
  represent themselves; that is, characters with octet values 0x20-0x25
  and 0x27-0x7e.  The character "&" (0x26) is represented by the two-
  octet sequence "&-".

  All other characters (octet values 0x00-0x1f, 0x7f-0xff, and all
  Unicode 16-bit octets) are represented in modified BASE64, with a
  further modification from [UTF-7] that "," is used instead of "/".
  Modified BASE64 MUST NOT be used to represent any printing US-ASCII
  character which can represent itself.

  "&" is used to shift to modified BASE64 and "-" to shift back to US-
  ASCII.  All names start in US-ASCII, and MUST end in US-ASCII (that
  is, a name that ends with a Unicode 16-bit octet MUST end with a "-
  ").

  For example, here is a mailbox name which mixes English, Japanese,
  and Chinese text: ~peter/mail/&ZeVnLIqe-/&U,BTFw-

This class will do the correct Encoding for the IMAP mailboxes.